What if all dark matter disappeared?

So…What happens?

The effect that the disappearance of dark matter would have on the universe would be substantial, but we will focus mainly on galaxies for our question. First, we will cover what would happen to stars within galaxies, and then how the effects might differ among different kinds of galaxies. For this thought experiment, we will assume that all dark matter suddenly disappears without turning into anything else.

Starting with stars, we can break these down into two groups: stars close to the center and stars in the outer regions. For this discussion, we will assume a typical large spiral galaxy like our own. Near the center, visible matter like stars and gas can make up most of the mass, even though dark matter is also present. Dark matter becomes increasingly important farther out because its roughly spherical halo extends well beyond the bright disk of stars. So what does this actually mean? Basically, the inner regions have more gravity coming from matter that would still be there after dark matter disappears. Many of their stars could remain bound, although their orbits would still change.

To understand why, we need to talk about escape velocity. Galaxies viewed from afar do not appear to be moving very quickly, but appearances can be deceiving. Stars orbit within them at incredible speeds, with our own Sun traveling around the Milky Way at roughly half a million miles per hour. Escape velocity is the minimum speed an object needs to escape a gravitational system without any additional push. If a star’s speed is below the escape velocity at its location, it remains gravitationally bound. If its speed exceeds that threshold, it can escape.

The familiar escape velocity equation was mentioned earlier, and it works for a simplified case, such as being outside a spherical object. Inside a galaxy, the calculation is more complicated because mass is spread throughout the galaxy and its halo. The basic idea still applies, though: removing mass weakens the gravitational pull and lowers the escape velocity. At the instant dark matter disappears, the stars retain their existing speeds, but some could now be moving fast enough to escape.

This would be especially important in the outer regions of a spiral galaxy, where visible matter is much more spread out and dark matter has a larger role. How do we know this unseen mass is there? In the 1970s, astronomer Vera Rubin and her collaborators, including Kent Ford, measured how quickly material orbited within spiral galaxies. If the galaxies contained only the observed visible matter, orbital speeds should generally have declined sufficiently far beyond the bright central regions. Instead, the measured speeds remained surprisingly high, producing what astronomers call flat rotation curves. This was powerful evidence that galaxies contained much more mass than we could see.

When dark matter disappears, those outer stars would initially keep moving at the same speeds, but there would be less gravity to hold them in the galaxy. Some could exceed the new escape velocity and move away into space. Imagine spinning a ball around on a string and then suddenly letting go. The ball does not need an extra push to fly away; it already has motion. The comparison is not perfect, because the galaxy’s remaining matter would still exert gravity, but it captures the main idea. Over millions of years and longer, escaping stars could spread out and the galaxy could lose part of its familiar structure. Exactly how many stars escape would depend on the galaxy’s mass distribution and their original motions.

There is still one more aspect we are leaving out, though. The stars that remain bound would not be totally unaffected. With less gravity pulling them inward, many would move onto wider or more elongated orbits. As the stars redistribute themselves, the galaxy’s gravitational field would continue to change, and the remaining system could expand before settling into a new balance. If the gravitational field changes rapidly enough during this readjustment, a process called “violent relaxation” can help redistribute the stars’ energies. Despite the dramatic name, this does not mean stars are smashing into each other. The remaining galaxy could become more spread out, though it would not necessarily turn into a sphere.

So how might this differ among other types of galaxies? Some elliptical galaxies have dense central regions where stars provide much of the gravity. Those regions could retain many of their stars after dark matter disappears. However, we cannot automatically say that elliptical galaxies would fare the best. Their dark matter content varies, and some contain substantial amounts even within their visible regions. Like spirals, they could lose loosely bound stars and expand as their remaining stars adjust to the weaker gravitational field. What matters most is how much of the gravity came from dark matter before it vanished.

For irregular galaxies, the outcome would also depend on what they contain. Their lack of a neat shape does not automatically mean they are barely held together, and a galaxy does not need one massive central object to bind it. Gravity comes from all of its matter together. Some irregular galaxies are small and strongly dominated by dark matter, which could make them especially vulnerable, but irregular shape alone does not tell us what would happen.

The clearest examples of galaxies in serious trouble would be dark-matter-dominated dwarf galaxies, whether they are irregular or more rounded. In extreme cases, dark matter can account for around 99% of a galaxy’s mass. Removing that much of the gravitational support could leave a large fraction of its stars unbound, causing the galaxy to disperse over time. Individual dense star clusters might still hold themselves together, even while the larger galaxy loses its structure.

So what would this mean for us? In everyday life, probably very little at first. Earth would still orbit the Sun because the Sun’s gravity dominates our solar system, and we would not suddenly see stars shooting across the night sky. The Sun’s path through the Milky Way could change, but noticeable changes in the arrangement of stars would unfold over enormous timescales.

Looking at other galaxies would introduce another delay: light takes time to reach us. Andromeda, our nearest large galactic neighbor, is about 2.5 million light-years away, although smaller galaxies are much closer. Under our assumption that dark matter disappears everywhere simultaneously, we would have to wait about 2.5 million years to receive the first light showing Andromeda after the disappearance. Its large-scale structural changes would then develop over millions to hundreds of millions of years, depending on which part of the galaxy we were watching.

Scientists would face another challenge: we do not currently have a confirmed dark matter particle signal that could simply switch off. Underground detectors search for rare interactions that might reveal dark matter, rather than recording a steady background hum from it. Its disappearance would therefore not automatically set off an alarm in those experiments. Astronomers would instead have to investigate changes in gravitational effects, such as stellar motions and the bending of light, as evidence became observable. Exactly how quickly they could figure out what had happened is difficult to predict, though we bet they would be very concerned when or if they do find out.