Asked by: Merlin Batchelor, Norwich
Some astronomers have indeed theorised that dark matter might just be ordinary matter that we cannot see, rather than an exotic, as-yet-undiscovered particle. This ordinary matter could include black holes, neutron stars, brown dwarfs, white dwarfs, very faint red dwarfs and even solitary planets.
These objects, collectively known as MACHOs (Massive Astrophysical Compact Halo Objects), emit very little light, but they can be detected if they pass in front of or near a background object (via the way that their gravity bends the light from the more distant object).
However, studies to date have concluded that MACHOs can only account for a tiny fraction of the missing mass in the Universe. So the nature of dark matter remains a mystery.