In astrophysics and particle physics, self-interacting dark matter (SIDM) is an alternative class of dark matter particles which have strong interactions, in contrast to the standard cold dark matter model (CDM). SIDM was postulated in 2000^[1] as a solution to the core-cusp^[2][3][4] problem. In the simplest models of DM self-interactions, a Yukawa-type potential and a force carrier φ mediates between two dark matter particles.^[5] On galactic scales, DM self-interaction leads to energy and momentum exchange between DM particles. Over cosmological time scales this results in isothermal cores in the central region of dark matter haloes.

If the self-interacting dark matter is in hydrostatic equilibrium, its pressure and density follow:

${\displaystyle \nabla P_{\chi }/\rho _{\chi }=\nabla \Phi _{\rm {tot}}=\nabla (\Phi _{\chi }+\Phi _{b})}$

where ${\displaystyle \Phi _{\chi }}$ and ${\displaystyle \Phi _{b}}$ are the gravitational potential of the dark matter and a baryon respectively. The equation naturally correlates the dark matter distribution to that of the baryonic matter distribution. With this correlation, the self-interacting dark matter can explain phenomena such as the Tully–Fisher relation.

Self-interacting dark matter has also been postulated as an explanation for the DAMA annual modulation signal.^[6][7][8] Moreover, it is shown that it can serve the seed of supermassive black holes at high redshift.^[9] SIDM may have originated in a so-called "Dark Big Bang"^[10]

• MACS J0025.4-1222, astronomical observations that constrain DM self-interaction
• ESO 146-5, the core of Abell 3827 that was claimed as the first evidence of SIDM
• Strongly interacting massive particle (SIMP), proposed to explain cosmic ray data
• Lambda-CDM model