Brown dwarf. Artist's impression of what a brown dwarf star" might look like. A brown dwarf is a hypothetical planet that is enormous but not quite big enough to self-ignite and turn into a star. Because they do not shine brown dwarfs are invisi- ble, and hence there is no direct evidence that they exist. In spite of this many astronomers believe that there are millions of brown dwarfs throughout Space, and that this accounts for some of the missing "dark matter" that is thought to make up 90% of the Universe."