In order to explain how to make the universe run smoothly, theories suggest that all galaxies should be filled with a large amount of dark matter. However, over the years, scientists have discovered several galaxies with almost no dark matter, which seriously challenges the rules of the universe. Fortunately, a group of teams recently identified how dark matter-free galaxies can survive in a universe that contains a lot of dark matter — the former has lost track of dark matter.
Although dark matter has always been invisible, there are many clues to guide the way. The first and earliest evidence is galaxies. Observational data show that the rotation speed of the outer side of the galaxy is faster than that expected by Newton’s gravity, indicating that there must be a huge amount of mass energy pulling the outer side of the galaxy to prevent the galaxy from falling apart. Since then, dark matter has gained a firm foothold and is difficult to separate from the galaxy. Models suggest that dark matter condenses into filaments and forms nodes at higher densities, which in turn attract visible matter to gather to form clusters of galaxies, which, because galaxies contain much dark matter, continue to merge to shape galaxy clusters today.
Therefore, it is difficult to understand that a galaxy does not contain dark matter.
However, in 2018, scientists encountered an extremely embarrassing moment: the discovery of a galaxy with almost no dark matter, called NGC 1052-DF2. Because NGC 1052-DF2 is located near a group of galaxies dominated by the large elliptical galaxy NGC 1052, the team suspected at the time that billions of years ago, the giant fledgling elliptical galaxy might have somehow caused NGC 1052-DF2 during its growth. Galaxies lack dark matter.
Another idea is that the galaxy NGC 1052-DF2 experienced a catastrophic event inside that wiped out all the gas and dark matter and prevented new stars from forming.
A year later, in 2019, another team discovered a second galaxy without dark matter, named NGC 1052-DF4 (DF4 for short), proving that NGC 1052-DF2 is not a special case, and there must be an unimaginable dark matter-free galaxy in the universe Galaxies are waiting to be excavated.
New galaxy evolution model unexpectedly confirms past inferences
Recently, the University of California, Irvine team originally wanted to model the evolution of galaxies, but the model unexpectedly produced 7 galaxies with almost no dark matter. The researchers then used the model to examine the evolution of these galaxies and found that at first, these galaxies had no dark matter. Dark matter galaxies are actually normal small galaxies, and dark matter also exists, but they are eventually stripped of dark matter due to mergers with larger nearby galaxies.
In short, these small galaxies were engulfed, but somehow reappeared on the other side with about half of the stars, but with all the gas and dark matter missing.
The theory confirms the team’s idea in 2018 that collisions between neighboring galaxies would trigger looting, leaving smaller galaxies with only a few stars and remnant dark matter left.
The University of California, Irvine team said that this also shows that the galaxies lacking dark matter should be more than the two discovered above, especially in the vicinity of massive galaxies. As many as 30% of the massive galaxies in the universe may have small galaxies without dark matter.
Astronomers are busy with new topics. The research paper was published in the journal Nature Astronomy.