Dwarf galaxy problem

The dwarf galaxy problem, also known as the missing satellites problem, arises from a mismatch between observed dwarf galaxy numbers and collisionless numerical cosmological simulations that predict the evolution of the distribution of matter in the universe.

However, although there seem to be enough observed normal-sized galaxies to match the simulated distribution of dark matter halos of comparable mass, the number of observed dwarf galaxies is orders of magnitude lower than expected from such simulation.

[3][4] There are two main alternatives which may resolve the dwarf galaxy problem: The smaller-sized clumps of dark matter may be unable to obtain or retain the baryonic matter needed to form stars in the first place; or, after they form, dwarf galaxies may be quickly “eaten” by the larger galaxies that they orbit.

In support of this, in 2007 the Keck telescopes observed eight newly discovered ultra-faint Milky Way dwarf satellites of which six were around 99.9% dark matter (with a mass-to-light ratio of about 1,000).

Tidal stripping may also have been part of the problem of detecting dwarf galaxies in the first place: Finding dwarf galaxies is an extremely difficult task, since they tend to have low surface brightness and are highly diffuse – so much so that they are close to blending into background and foreground stars.