If you look across space with a telescope, you’ll see countless galaxies, most of which host large central black holes, billions of stars and their attendant planets. The universe teems with huge, spectacular objects, and it might seem like these massive objects should hold most of the universe’s matter.

But the Big Bang theory predicts that about 5% of the universe’s contents should be atoms made of protons, neutrons and electrons. Most of those atoms cannot be found in stars and galaxies – a discrepancy that has puzzled astronomers.

If not in visible stars and galaxies, the most likely hiding place for the matter is in the dark space between galaxies. While space is often referred to as a vacuum, it isn’t completely empty. Individual particles and atoms are dispersed throughout the space between stars and galaxies, forming a dark, filamentary network called the “cosmic web.”

Throughout my career as an astronomer, I’ve studied this cosmic web, and I know how difficult it is to account for the matter spread throughout space.

In a study published in June 2025, a team of scientists used a unique radio technique to complete the census of normal matter in the universe.

The census of normal matter

The most obvious place to look for normal matter is in the form of stars. Gravity gathers stars together into galaxies, and astronomers can count galaxies throughout the observable universe.

The census comes to several hundred billion galaxies, each made of several hundred billion stars. The numbers are uncertain because many stars lurk outside of galaxies. That’s an estimated 10²³ stars in the universe, or hundreds of times more than the number of sand grains on all of Earth’s beaches. There are an estimated 10⁸² atoms in the universe.

However, this prodigious number falls far short of accounting for all the matter predicted by the Big Bang. Careful accounting indicates that stars contain only 0.5% of the matter in the universe. Ten times more atoms are presumably floating freely in space. Just 0.03% of the matter is elements other than hydrogen and helium, including carbon and all the building blocks of life.

Looking between galaxies

The intergalactic medium – the space between galaxies – is near-total vacuum, with a density of one atom per cubic meter, or one atom every 35 cubic feet. That’s less than a billionth of a billionth of the density of air on Earth. Even at this very low density, this diffuse medium adds up to a lot of matter, given the enormous, 92-billion-light-year diameter of the universe.

The intergalactic medium is very hot, with a temperature of millions of degrees. That makes it difficult to observe except with X-ray telescopes, since very hot gas radiates out through the universe at very short X-ray wavelengths. X-ray telescopes have limited sensitivity because they are smaller than most optical telescopes.

Deploying a new tool

Astronomers recently used a new tool to solve this missing matter problem. Fast radio bursts are intense blasts of radio waves that can put out as much energy in a millisecond as the Sun puts out in three days. First discovered in 2007, scientists found that the bursts are caused by compact stellar remnants in distant galaxies. Their energy peters out as the bursts travel through space, and by the time that energy reaches the Earth, it is a thousand times weaker than a mobile phone signal would be if emitted on the Moon, then detected on Earth.

Research from early 2025 suggests the source of the bursts is the highly magnetic region around an ultra-compact neutron star. Neutron stars are incredibly dense remnants of massive stars that have collapsed under their own gravity after a supernova explosion. The particular type of neutron star that emits radio bursts is called a magnetar, with a magnetic field a thousand trillion times stronger than the Earth’s.

Even though astronomers don’t fully understand fast radio bursts, they can use them to probe the spaces between galaxies. As the bursts travel through space, interactions with electrons in the hot intergalactic gas preferentially slow down longer wavelengths. The radio signal is spread out, analogous to the way a prism turns sunlight into a rainbow. Astronomers use the amount of spreading to calculate how much gas the burst has passed through on its way to Earth.

Puzzle solved

In the new study, published in June 2025, a team of astronomers from Caltech and the Harvard Center for Astrophysics studied 69 fast radio bursts using an array of 110 radio telescopes in California. The team found that 76% of the universe’s normal matter lies in the space between galaxies, with another 15% in galaxy halos – the area surrounding the visible stars in a galaxy – and the remaining 9% in stars and cold gas within galaxies.

The complete accounting of normal matter in the universe provides a strong affirmation of the Big Bang theory. The theory predicts the abundance of normal matter formed in the first few minutes of the universe, so by recovering the predicted 5%, the theory passes a critical test.

Several thousand fast radio bursts have already been observed, and an upcoming array of radio telescopes will likely increase the discovery rate to 10,000 per year. Such a large sample will let fast radio bursts become powerful tools for cosmology. Cosmology is the study of the size, shape and evolution of the universe. Radio bursts could go beyond counting atoms to mapping the three-dimensional structure of the cosmic web.

Pie chart of the universe

Scientists may now have the complete picture of where normal matter is distributed, but most of the universe is still made up of stuff they don’t fully understand.

The most abundant ingredients in the universe are dark matter and dark energy, both of which are poorly understood. Dark energy is causing the accelerating expansion of the universe, and dark matter is the invisible glue that holds galaxies and the universe together.

Dark matter is probably a previously unstudied type of fundamental particle that is not part of the standard model of particle physics. Physicists haven’t been able to detect this novel particle yet, but we know it exists because, according to general relativity, mass bends light, and far more gravitational lensing is seen than can be explained by visible matter. With gravitational lensing, a cluster of galaxies bends and magnifies light in a way that’s analogous to an optical lens. Dark matter outweighs conventional matter by more than a factor of five.

One mystery may be solved, but a larger mystery remains. While dark matter is still enigmatic, we now know a lot about the normal atoms making up us as humans, and the world around us.

Chris Impey, University Distinguished Professor of Astronomy, University of Arizona

This article is republished from The Conversation under a Creative Commons license. Read the original article.

The committee advising the Centers for Disease Control and Prevention on vaccine policy voted on Dec. 5, 2025, to stop recommending that all newborns be routinely vaccinated against the hepatitis B virus – undoing a 34-year prevention strategy that has nearly eliminated early childhood hepatitis B infections in the United States.

Before the U.S. began vaccinating all infants at birth with the hepatitis B vaccine in 1991, around 18,000 children every year contracted the virus before their 10th birthday – about half of them at birth. About 90% of that subset developed a chronic infection.

In the U.S., 1 in 4 children chronically infected with hepatitis B will die prematurely from cirrhosis or liver cancer.

Today, fewer than 1,000 American children or adolescents contract the virus every year – a 95% drop. Fewer than 20 babies each year are reported infected at birth.

I am a pediatrician and preventive medicine specialist who studies vaccine delivery and policy. Vaccinating babies for hepatitis B at birth remains one of the clearest, most evidence-based ways to keep American children free of this lifelong, deadly infection.

What spurred the change?

In September 2025, the Advisory Committee on Immunization Practices, or ACIP, an independent panel of experts that advises the CDC, debated changing the recommendation for a dose of the hepatitis B vaccine at birth, but ultimately delayed the vote.

This committee regularly reviews vaccine guidance. However, since Secretary of Health and Human Services Robert F. Kennedy Jr. disbanded the entire committee and handpicked new members, its activity has drastically departed from business as usual. The committee has long-standing procedures for evaluating evidence on the risks and benefits of vaccines, but these procedures were not followed in the September meeting and were not followed for this most recent decision.

The committee’s new recommendation keeps the hepatitis B vaccine at birth for infants whose mothers test positive for the virus. But the committee now advises that infants whose mothers test negative should consult with their health care provider. Parents and health care providers are instructed to weigh vaccine benefits, vaccine risks and infection risks using “individual-based decision-making” or “shared clinical decision-making.”

On the surface, this sounds reasonable. But while parents have always been free to discuss benefits and risks with their health care providers to make a decision on what’s best for their child, this change is not based on any new evidence, and it introduces uncertainty into a recommendation that has long been clear.

As a doctor, I am already seeing this uncertainty play out in the clinic. I recently had new parents ask to postpone the hepatitis B vaccine until adolescence because they believed federal health leaders had evidence that people only become infected through sexual activity or contaminated needle use.

After a brief conversation, they came to understand that this was inaccurate — children can be infected not only at birth but also through routine household or child care exposures, including shared toothbrushes or even a bite that breaks the skin. In the end, they chose to vaccinate, but this experience highlights how easily well-intentioned parents can be misled when guidance is not clear and consistent.

Why the CDC adopted universal hepatitis B shots

Hepatitis B is a virus that infects liver cells, causing inflammation and damage. It is spread through blood and bodily fluids and is easily transmitted from mother to baby during delivery.

The hepatitis B vaccine has been available since the early 1980s. Before 1991, public health guidance recommended giving newborns the hepatitis B vaccine only if they were at high risk of being infected – for example, if they were born to a mother infected with hepatitis B.

That targeted plan failed. Tens of thousands of infants were still infected each year.

Some newborns were exposed when their mothers weren’t screened; others were exposed after their mothers were infected late in pregnancy, after their initial screening. And like any lab test, the screening can have false negative results, be misinterpreted or not be communicated properly to the baby’s care team.

Recognizing these gaps, in 1991 the CDC recommended hepatitis B vaccination for every child starting at birth, regardless of maternal risk.

The U.S. adopted a policy of vaccinating all babies from birth because the number of people with hepatitis B infections was, and remains, relatively high, and because many mothers do not receive prenatal care, so their infections go undetected.

Meanwhile, in some European countries, like Denmark, only babies with certain risk factors receive the vaccine at birth. That’s because in those countries, hepatitis B infections are much less prevalent and pregnant mothers are more widely tested due to universal health care. Due to these differences, that approach is not effective in the United States. In fact, most World Health Organization member countries do recommend a universal birth dose.

Vaccinating at birth

The greatest danger for infants contracting hepatitis B is at birth, when contact with a mother’s blood can transmit the virus. Without preventive treatment or vaccination, 70% to 90% of infants born to infected mothers will become infected themselves, and 90% of those infections will become chronic. The infection in these children silently damages their liver, potentially leading to liver cancer and death.

About 80% of parents choose to vaccinate their babies at birth. If parents choose to delay vaccination due to this new recommendation, it will leave babies unprotected during this most vulnerable window, when infection is most likely to lead to chronic infection and silently damage the liver.

A research article published on Dec. 3, 2025, estimates that if only infants born to mothers infected with hepatitis B received the vaccine, an additional 476 perinatal hepatitis B infections would occur each year.

The hepatitis B vaccines used in the U.S. have an outstanding safety record. The only confirmed risk is an allergic reaction called anaphylaxis that occurs in roughly 1 in 600,000 doses, and no child has died from such a reaction. Extensive studies show no link to other serious conditions.

How children get exposed to hepatitis B

Infants and children continue to be vulnerable to hepatitis B long after birth.

Children can become infected through household contacts or in child care settings by exposures as ordinary as shared toothbrushes or a bite that breaks the skin. Because hepatitis B can survive for a week on household surfaces, and many carriers are unaware they are infected, even babies and toddlers of uninfected mothers remained at risk.

Full protection against hepatitis B requires a three-dose vaccine series, given at specific intervals in infancy. Anything short of the full series leaves children vulnerable for life.

In addition to changing the birth dose recommendation, the committee is now advising parents to consult with their health care provider about checking children’s antibody levels after one or two doses of the vaccine to determine whether additional doses are needed. While such testing is sometimes recommended for people in high-risk groups after they get all three doses to confirm their immune system properly responded to the vaccine, it is not a substitute for completing the series.

The recommendation for all babies to receive the vaccine at birth and for infants to complete the full vaccine series is designed to protect every child, including those who slip through gaps in maternal screening or encounter the virus in everyday life. A reversion to the less effective risk-based approach threatens to erode this critical safety net.

Portions of this article originally appeared in a previous article published on Sept. 9, 2025.

David Higgins, Assistant Professor of Pediatrics, University of Colorado Anschutz Medical Campus

This article is republished from The Conversation under a Creative Commons license. Read the original article.