Dark Matter
A fundamental question scientists have been asking about the expanding universe is whether it will continue to expand forever. Since the big bang occurred billions of years ago, galaxies formed, evolved, and have been flying deeper and deeper into space. If there is enough total mass in the universe, its gravitational attraction will slow the expansion, reverse it, and eventually result in a "big crunch." If the mass is insufficient, the expansion could continue forever. Much of the total mass of the universe may not be detectable by ordinary instruments such as a telescope--hence the search for dark matter which would tell cosmologists where the universe is headed.
In 1933 Swiss astronomer Fritz Zwicky observed that some galaxies in a cluster had a higher speed than expected. It was surmised that invisible dark matter was gravitationally affecting their movement. His measurements showed there had to be more than ten times as much invisible matter as visible matter in this cluster to account for the higher speed. This was considered so outlandish that few astronomers accepted it. But in the 1970s other clues pointing toward the existence of dark matter began appearing.
At that time, a study of sixty galaxies made at the Carnegie Institution in Washington, D.C., found that stars at the perimeter of the galaxies are moving at nearly the same speed as those near the galactic cores. The galaxies all seem to be surrounded by halos of invisible matter which make the outer stars move faster.
In 1987 American astronomers discovered faint arcs of light around the centers of two galaxy clusters. It was suggested the arcs are from a single object lying beyond the two clusters, and dark mass in the galaxy clusters (amounting to sixty times the visible mass) is acting like a gravitational lens, bending the light into two arcs. Of what, then, is the dark matter composed? Black holes have been suggested as the invisible matter responsible, but scientists don't think there are enough of these to account for the mass required to explain their observations.
In fact, most scientists think that some dark matter is not composed of atoms. While ordinary matter such as dust or gas leaves traces by either emitting, deflecting, or absorbing electromagnetic radiation, the matter presumed to be surrounding galaxies does not--it can only be "seen" by its gravitational pull on visible objects. Exotic particles called neutrino s, increasingly believed to be common in the universe, may have the characteristics required. These and other so-called weakly interacting massive particles (WIMPs) and axions are possible dark matter candidates, but their existence has yet to be proven, and extraordinary means will be required to detect them.
This is the complete article, containing 442 words
(approx. 1 page at 300 words per page).
