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.
as written by the authors arXiv:2603.10107
Dark matter (DM) could be subject to non-gravitational self-interactions which is relevant to resolve potential problems of cold DM on small scales. Their impact on astrophysical objects such as galaxies and galaxy clusters allows for constraining the strength of this scattering and eventually further properties of the cross-section. To model self-interacting dark matter (SIDM), N-body simulations are a crucial tool widely employed by the SIDM community. In this paper, we describe the SIDM implementation in the cosmological hydrodynamical N-body code OpenGadget3 and release it to the public. It is capable of simulating elastic scattering for various differential cross-sections, including strongly anisotropic cross-sections. Beyond single-species models, the code also allows simulating a two-species model with cross-species interactions. In addition to describing the numerical schemes for modelling various flavours of SIDM, we discuss the technical challenges of implementing them. Moreover, we demonstrate through several test problems that OpenGadget3 can accurately simulate DM self-interactions. Furthermore, we assess the performance of the code and provide scaling tests. Lastly, we highlight remaining challenges in the context of SIDM and describe directions for improving the current state of the art.
There is about five times more dark matter in the universe than ordinary matter, which makes up stars, planets, and ourselves.