Now evidence suggests that the galaxy is enveloped by a huge sphere of dark matter greater than 300,000 light-years across. This means that about eighty percent of our galaxy cannot be seen. Other galaxies spin so fast that were it not for the weight of some unseen mass they would fly apart. Clusters of galaxies can only be explained if the weight of the cluster is more than that of the bright galaxies it contains. The dark matter theory holds that gravity amplified tiny fluctuations in the distribution of matter in the early universe to produce vast fields of galaxies.
Dark matter consists as an unidentified substance that interacts only slightly with ordinary matter.
Such particles need to be heavy to provide enough gravitational attraction, and they need to be very aloof. If dark matter took part in chemical or nuclear reactions, their presence would be all too obvious. One candidate for dark matter is thought to be normal atomic matter tied up in objects smaller than stars but larger than most planets. There could be thousands of dark objects for every star in the galaxy. Called baryonic matter, it consists of large planets, low-mass stars, white dwarfs, neutron stars, or even black holes.