Dark matter

Dark matter is a component of the universe whose presence is discerned from its gravitational attraction rather than its luminosity. Dark matter makes up 30.1 per cent of the matter-energy composition of the universe; the rest is dark energy (69.4 per cent) and “ordinary” visible matter (0.5 per cent).

Originally known as the “missing mass,” dark matter’s existence was first inferred by Swiss American astronomer Fritz Zwicky, who in 1933 discovered that the mass of all the stars in the Coma cluster of galaxies provided only about 1 per cent of the mass needed to keep the galaxies from escaping the cluster’s gravitational pull.

The reality of this missing mass remained in question for decades, until the 1970s when American astronomers Vera Rubin and W. Kent Ford confirmed its existence by the observation of a similar phenomenon: the mass of the stars visible within a typical galaxy is only about 10 per cent of that required to keep those stars orbiting the galaxy’s centre.

In general, the speed with which stars orbit the centre of their galaxy is independent of their separation from the centre; indeed, orbital velocity is either constant or increases slightly with distance rather than dropping off as expected. To account for this, the mass of the galaxy within the orbit of the stars must increase linearly with the distance of the stars from the galaxy’s centre. However, no light is seen from this inner mass—hence the name “dark matter.”

Since the confirmation of dark matter’s existence, a preponderance of dark matter in galaxies and clusters of galaxies has been discerned through the phenomenon of gravitational lensing—matter acting as a lens by bending space and distorting the passage of background light. The presence of this missing matter in the centres of galaxies and clusters of galaxies has also been inferred from the motion and heat of gas that gives rise to observed X-rays.

For example, the Chandra X-ray Observatory has observed in the Bullet cluster, which consists of two merging galaxy clusters, that the hot gas (ordinary visible matter) is slowed by the drag effect of one cluster passing through the other. The mass of the clusters, however, is not affected, indicating that most of the mass consists of dark matter.

Matter is 30.6 per cent of the universe’s matter-energy composition. Only 0.5 per cent is in the mass of stars and 0.03 per cent of that matter is in the form of elements heavier than hydrogen. The rest is dark matter. Two varieties of dark matter have been found to exist. The first variety is about 4.5 per cent of the universe and is made of the familiar baryons (i.e., protons, neutrons, and atomic nuclei), which also make up the luminous stars and galaxies.

Most of this baryonic dark matter is expected to exist in the form of gas in and between the galaxies. This baryonic, or ordinary, component of dark matter has been determined by measuring the abundance of elements heavier than hydrogen that were created in the first few minutes after the big bang occurred 13.8 billion years ago.

Britannica