Dark matter can refer to any substance which interacts predominantly via gravity with visible matter (e.g., stars and planets). Hence in principle it need not be composed of a new type of fundamental particle but could, at least in part, be made up of standard baryonic matter, such as protons or neutrons. Most of the ordinary matter familiar to astronomers, including planets, brown dwarfs, red dwarfs, visible stars, white dwarfs, neutron stars, and black holes, fall into this category. Solitary black holes, neutron stars, burnt-out dwarfs, and other massive objects that are hard to detect are collectively known as MACHOs ; some scientists initially hoped that baryonic MACHOs could account for and explain all the dark matter.