Missing satellites problem

The discrepancy between the many small dark-matter halos predicted around large galaxies by cosmological simulations and the smaller number of observed satellite galaxies.

Cold dark matter: A cosmological model in which most dark matter consists of slowly moving, nonrelativistic particles. Its small-scale structure predictions supply the abundant low-mass halos at the center of the problem.
Aquarius Project: A suite of high-resolution simulations of Milky Way-sized dark-matter halos. Its resolved subhalo populations sharpened predictions for the Milky Way's expected satellites.
Milky Way: The barred spiral galaxy that contains the Solar System. Its satellite census is the original and most extensively studied setting for the discrepancy.
Warm dark matter: A dark-matter model with particles moving fast enough to suppress the formation of small-scale structures. It reduces the predicted abundance of small halos rather than merely hiding their galaxies.
DESI Legacy Imaging Surveys: A set of wide-area optical imaging surveys covering much of the extragalactic sky. Its imaging supports searches for faint satellites beyond the Milky Way's immediate surroundings.
Dark matter halo: A gravitationally bound region of dark matter surrounding a galaxy or galaxy cluster. The predicted satellites are low-mass halos, not necessarily luminous galaxies.
Via Lactea: A high-resolution cosmological simulation of a Milky Way-sized dark-matter halo. It provided an influential independent prediction of numerous small subhalos.
Andromeda Galaxy: The nearest large spiral galaxy to the Milky Way. Its satellite population tests whether the mismatch extends beyond our own galaxy.
Self-interacting dark matter: A dark-matter model in which dark-matter particles scatter off one another. It changes halo structure through particle interactions, offering a different small-scale remedy.
Satellite luminosity function: The distribution of satellite galaxies as a function of their luminosity. Its faint end measures how many low-luminosity satellites remain to be found.

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