A class of galaxies discovered in 2018 are hard to explain via the most popular theory describing the origin of the universe. These galaxies are made solely of ordinary matter and contain no exotic components. Astronomers are discovering how these cosmic curiosities came to be, including a recent study that successfully simulated their creation.

Modern cosmology imagines that our universe is an astronomical confection, consisting of three primary ingredients: ordinary matter, dark matter, and dark energy. Ordinary matter consists of all of the stars and galaxies and gas that astronomers can see with their telescopes. Dark matter and dark energy have not been directly observed, but they seem to make up 95% of the matter and energy of the universe.

The effect of dark energy is thought to be the reason that the expansion of the universe is accelerating, but dark matter is more similar to ordinary matter. While dark matter doesn’t glow or emit electromagnetic radiation of any kind, it experiences gravity just like ordinary matter – it clumps and can be found surrounding most galaxies. Indeed, there appears to be about five times more dark matter than ordinary matter. When dark matter surrounds a galaxy, it causes the visible galaxy to spin more quickly. This effect has been observed in essentially all galaxies. Dark matter is an explanation that is favored by most astronomers.

However, dark matter hasn’t been directly observed and there are scientists who claim that the explanation for quickly spinning galaxies is that we either don’t understand gravity or we don’t understand the nature of motion. The overarching term for this kind of explanation is MOND, short for Modifications Of Newtonian Dynamics.

Because dark matter has not yet been directly observed, there are researchers who favor the MOND explanation and scientists look for definitive evidence that either dark matter or MOND is the explanation. Very strong evidence for the dark matter solution was found in 2018 when two dwarf galaxies were observed which showed no evidence that they contained any dark matter.

These galaxies, called NGC 1052-DF2 and NGC 1052-DF4, are ultra-diffuse dwarf galaxies located in proximity to the elliptical galaxy NGC 1052. Both of these dwarf galaxies are small, containing only about a hundred million stars. (In comparison, the Milky Way galaxy contains over a thousand times more.) Both DF2 and DF4 rotate at rates that are identical to what astronomers would predict if Newton’s laws of both motion and gravity are correct, using only the mass of the galaxies found in stars and gas. In short, these two galaxies appear to have no dark matter in them.

It may seem ironic, but the existence of galaxies with no dark matter is strong evidence that dark matter is real. This is because if MOND were the correct explanation for the rapid rotation of most galaxies, it’s hard to understand how a galaxy could NOT rotate quickly. On the other hand, if dark matter is real, perhaps it would be possible to separate a galaxy’s visible mass (e.g. stars and gas) and invisible mass (e.g. dark matter). Thus, a galaxy without dark matter means that dark matter is likely to be true.

The question then becomes “how can a dwarf galaxy come into existence without dark matter?” This is especially interesting, since standard theory suggests that dwarf galaxies should have MORE dark matter than most galaxies. In the standard theory, a dwarf galaxy containing both ordinary matter and dark matter can move through intergalactic space, interacting with intergalactic gas. That intergalactic “wind” can strip away hydrogen gas inside the dwarf galaxy, thus increasing the dark matter fraction, not reducing it.

Astronomers have proposed a method for stripping away dark matter instead. Start with two large galaxies, each surrounded with a cloud of dwarf galaxies. Have the two large galaxies collide. These collisions can generate tidal forces in the surrounding dwarf galaxies strong enough to separate the ordinary matter and dark matter. 

In a recent paper, scientists simulated the behavior of dwarf galaxies in a crowded galactic environment and found that they could predict the existence of dwarf galaxies like DF2 and DF4, supporting the dark matter hypothesis. 

There are a few researchers who dispute that DF2 and DF4 are ordinary-matter-only galaxies, and thus the debate continues. However, assuming that the original measurements of DF2 and DF4 are correct, this new simulation provides additional evidence that dark matter, and not MOND, is a correct part of the theory that governs the universe. This simulation might be a little piece of the biggest picture of all.