Star formation across the universe has been declining for billions of years, and the decline has been steep. What makes it a genuine puzzle is that the raw material has not run out. There is still plentiful hydrogen. Galaxies are simply making far fewer stars out of it than they once did.

How anyone knows this

Looking further away means looking further back, because light takes time to travel. A galaxy observed at a great distance is seen as it was when the light left it. Surveying galaxies at a range of distances therefore produces a record of how the universe behaved at different ages.

Doing this consistently shows that the rate at which stars formed peaked several billion years after the Big Bang and has fallen substantially since. Astronomers sometimes describe the present universe as being in a long decline from that peak.

Measuring the gas is harder than measuring the stars. Stars are bright; cold hydrogen mostly is not, and detecting it requires radio observations and careful inference. That difficulty is part of why the puzzle took so long to sharpen — for a while the simplest explanation, that galaxies were running out of fuel, could not be ruled out.

Why gas alone is not enough

A star forms when a cloud of gas collapses under its own gravity. For that to happen, the cloud must be cold and dense. Warm gas has too much internal motion; its particles are moving fast enough to resist collapse.

So the presence of hydrogen in a galaxy does not mean the galaxy can make stars from it. The gas has to be in the right condition and in the right place, and several processes work against that.

Explanations under investigation include the following, and they are complementary rather than competing.

Feedback from black holes. Material falling toward a supermassive black hole at a galaxy’s centre releases enormous energy, which can heat surrounding gas or drive it outward. A galaxy can, in effect, disrupt its own star formation.

Feedback from stars themselves. Supernovae and stellar winds inject energy into the surrounding medium, heating gas and pushing it away from regions where it might otherwise collapse.

Where the gas sits. Much of a galaxy’s hydrogen may be in a diffuse halo rather than in the dense disc, too spread out to collapse and slow to fall inward.

Fewer mergers. Galaxy collisions compress gas and trigger bursts of star formation. As the universe expanded, galaxies encountered each other less often.

What is not settled

The relative importance of these mechanisms, and how it varies with galaxy size and environment. Black hole feedback is generally thought to dominate in the largest galaxies and stellar feedback in smaller ones, but the boundary is contested and the details are the subject of active work.

There is also a methodological caution worth carrying. Much of what is known about cold gas at great distances rests on indirect tracers, since the hydrogen itself is difficult to detect directly at those ranges. Conclusions therefore depend on assumptions about how well the tracer stands in for the thing being traced, and those assumptions are periodically revised.

Why it is worth knowing

Beyond the specific question, this is a good example of how the shape of a scientific problem changes. The observation “star formation is declining” invites an obvious explanation — the fuel is running out — which turns out to be wrong. The real question becomes why abundant material is not being used, which is a question about process rather than supply.

Nothing here has any practical consequence on human timescales. The decline unfolds over billions of years, and the Sun will continue as it has. It is simply one of the larger facts about the universe we live in, and it is not the fact most people would guess.

Sources

• ScienceDaily reporting on research into the decline of star formation despite abundant hydrogen, September 2026
• Published survey literature on the cosmic star formation history
• Astrophysical literature on active galactic nucleus feedback and stellar feedback
• Radio astronomy work on neutral hydrogen and molecular gas tracers in distant galaxies