Galaxies DF2 and DF4 are peculiar collections of stars. They appear to be devoid of dark matter, something that shouldn't happen according to our formation mechanism for galaxies. The team that discovered them now believes they know how they formed. They are the product of a complicated cosmic collision between two galaxies.

As reported in the journal Nature, the model suggests that 8 billion years ago, two dwarf galaxies orbiting their larger companion NGC 1052 collided at a speed of about 300 kilometers (185 miles) per second. The high-velocity impact saw the two galaxies pass through one another and was also able to separate the dark matter component from the gas.

This resulted in a trail of ultra-diffuse galaxies as large as the Milky Way but with a lot fewer stars and significantly less dark matter than its progenitors. The team believes that part of the original dwarf galaxies – now richer in dark matter – are still out there, with the dark-matter free ones spread between them.

Galaxies DF2 and DF4 would be among these post-merger trail galaxies. The simulation is certainly intriguing and manages to explain why these two fascinating collections of stars have about 100 times less dark matter than the theory for galaxy formation would allow.

And this is why the discovery of these galaxies was received with both skepticism and interest. This formation scenario might help explain their formation but ultimately their nature begs a crucial question in astrophysics about the nature of dark matter.

We simply do not know what it is. Dark matter is the best theory we have to explain decades of observations but as many astrophysicists will tell you, it’s also pretty bad. The reason is that nobody has been able to detect what dark matter actually is. Our knowledge of it suggests that it only interacts gravitationally with regular matter. That’s why it could be separated from galaxies during a high-speed collision.

There are alternatives to dark matter, such as Modified Newtonian Dynamics (MOND), which suggests that some laws of physics are wrong and need to be modified to explain what we see in the universe. The existence of dark-matter free galaxies challenges theories such as MOND because it suggests that dark matter is indeed a substance, something that galaxies may or may not have.

Understanding objects such as DF2 and DF4 is important when it comes to our cosmological theories and the role dark matter plays in them. Dark matter needs to be lost or gained in mergers, and shapes the evolution of these galaxies either by being present or absent.