Background radiation has recently provided some strong constraints on
conventional candidates for dark matter, which leads us to suggest ano
ther possibility: Kaluza-Klein solitons. These are the analogs of blac
k holes in the simplest extension of general relativity from four to f
ive dimensions, a theory which is otherwise in agreement with astrophy
sical data. We outline the essential properties of such solitons, whic
h resemble balls of radiation-like matter. Information on solitons can
be obtained from background radiation, the morphologies of galaxies,
and gravitational lensing.