Most cosmologists believe that dark matter, the mysterious substance that seems to account for six-sevenths of the matter in the Universe, exists in the form of myriad weakly interacting massive particles, or WIMPs.
The WIMPs won out over ideas of a cosmos filled with planets or black holes (collectively known, appropriately, as MACHOs, Massive Compact Halopassing MACHO on a background star; if the alignment was perfect, gravitational lensing would cause the background star to brighten and then fade in a predictable pattern.