Dark matter in astronomy

Evidence for dark matter in the outer parts of galaxies is found both in extended rotation curves and in the relative motions of binary galaxies. The best evidence for dark matter remains the dynamics of the great clusters of galaxies, i.e. where galaxies are most common. The lack of good evidence for dark matter in dwarf spheroidal galaxies means that any deductions from them are most doubtful. All methods of assessing Omega from mass to light ratios give zero dark matter content to voids. Since much of the universe is empty, Omega could be much greater than the optical assessment of Omega between the values of 0.1 and 0.3. The IRAS high Omega is explained as a biasing of IRAS galaxies away from high-density regions.