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Need for Dark Matter

The matter we can see, in the form of stars and gas, is not enough to explain the motion of stars and galaxies. Most of the matter in the Universe is said to consist of 'dark matter'. The nature of dark matter is unknown, but there are several candidates.
Massive Astronomical Compact Halo Objects - MACHOs. There is some evidence that lots of ordinary matter does exist The outer reaches of galaxies, called halos, might exists of large numbers of low mass failed stars or high mass planets, which are too dim to be seen with telescopes, or even black holes.
Some fundamental particles are known to exist in huge numbers eg neutrinos. Their masses are not known, and may be zero. If the masses of these particles were not zero, these particles could account for all the missing mass and we not not need dark matter.
Weakly Interacting Massive Particles. There may be particles which interact hardly at all with ordinary matter, and are only detectable because of their gravitational effects.
We know the best theory of gravity that we have - the General Theory of Relativity is incomplete (and so is quantum physics). Another theory might emerge which unifies these two theories, and does away with the need for dark matter.