Dark matter is invisible, but its gravity holds galaxies together. For a long time, it was thought that dark matter particles pass right through each other without noticing. However, in small galaxies, the density of dark matter in the center turns out to be lower than calculations predict. As if the invisible particles sometimes collide and bounce off each other like billiard balls.
A team of scientists has built such collisions into the computer code OpenGadget3, which simulates the birth and growth of galaxies. Now the code can calculate how dark matter particles scatter off each other when they approach. You can set how strongly they scatter and in which directions they fly apart after the hit. It's like tuning the elasticity of invisible billiard balls.
The authors tested the accuracy on several problems with known answers. The results matched expectations, and the calculation speed remained high even with a large number of particles. There are still open questions: for example, how to model collisions where particles don't just bounce off but merge or exchange energy in other ways.
Now any researcher can download the code and test their hypotheses about dark matter. Perhaps these very collisions will explain the mysteries of small galaxies and bring us closer to understanding what a quarter of the universe is made of.
There is about five times more dark matter in the universe than ordinary matter, which makes up stars, planets, and ourselves.