MOdified Newtonian Dynamics (MOND)

Mordehai Milgrom (see Scientific American Aug. 2002) postulates a modification to Newtonian dynamics which reveals itself when accelerations are smaller than a given value that is proposed as a new constant of nature.

This modification is claimed to explain most features of galactic dynamics without recourse to dark matter, including the Tully-Fisher correlation between galaxy brightness and the orbital speeds of stars in the outskirts of spiral galaxies.

MACHOs & WIMPs

Hot dark matter moves at relativistic speeds. One candidate is the neutrino which is now known to have a small mass but insufficient to account for a significant amount of the missing mass.
MAssive Compact Halo Objects (MACHOs) and Weakly Interacting Massive Particles (WIMPs) are candidates for cold dark matter.
MACHOs include brown dwarfs, cooled white dwarfs, neutron stars, black holes and minor bodies such as planets, asteroids and comets. Measurements suggest that such material can account for no more than 20% of the Milky Way's dark matter halo. WIMPs have yet to be detected.
The search for dark matter continues but what of other possibilities? Is it correct to assume that stars in a galaxy, and galaxies in a cluster, are gravitationally bound? How would we know if they were not?

Gravitational lensing

Measurements of gravitational lensing confirm the values for masses derived from galaxy dynamics.

Mass-to-light ratios

The two types of mass measurement for galaxies give very different answers. Those based on the effects of gravity (dynamics of stars determined from measurements of redshift) are ten times greater than those inferred from brightness. The mass-to-light ratio for the Sun is 1; the mass-to-light ratio for galaxies is ~10. The mass-to-light ratio increases to ~100 when galactic halos are included; and rises to ~300 for clusters of galaxies.

Standard reasoning asserts that, since bright matter underestimates the amount of mass in a galaxy, this 'missing mass' must be present in a 'dark' form.

Occam's razor rules OK!

It seems to me that current theories of the Universe defy the principle of Occam's razor. Each new hypothesis is added to overcome the difficulties raised by the last and lacks supporting experimental evidence.