dark matter

dark matter

Matter that probably comprises 75% (or more) of the mass of the Universe but is undetectable except by its gravitational effects. Dark matter was first suspected in clusters of galaxies when the galaxies were found to move with too high a speed to be retained in the cluster by their gravitational influence on each other. The term missing mass was coined for the necessary invisible matter in the cluster, amounting to 10 to 100 times the amount of visible matter in the galaxies and in the intergalactic medium. In the 1970s investigations of the rotation of spiral galaxies indicated that these galaxies have a dark halo containing some 10 times as much matter as the visible galaxy. Simultaneously statistical analyses of the motions of galaxies on a larger scale than clusters suggested that the Universe in general has a ratio of perhaps 4:1 of dark matter to baryonic matter (except within galaxies, where ordinary matter dominates).

The nature of the dark matter is highly elusive, and there is no certainty even that the dark haloes of spiral galaxies consist of the same matter as the missing mass of clusters and the Universe at large. A proportion may consist of objects composed of normal matter, but according to standard Big-Bang models the whole of the missing mass cannot consist of matter containing baryons (protons and neutrons) since that amount of baryonic matter would have produced a higher abundance of helium than is observed. The bulk of the dark matter does not therefore consist of dim stars or smaller chunks of solid matter, nor of black holes that have formed from stars.

If the dark matter candidate was moving at velocities comparable to the speed of light as it decoupled from the early Universe, it would subsequently smear out all early fluctuations except on the very largest scales (see microwave background radiation). This candidate, known as hot dark matter, thus predicts that superclusters of galaxies were the first structures to form, fragmenting later to form bound clusters and galaxies in a top-down scenario. This theory is no longer in favor because it has problems with the formation of galaxies at high redshifts, and is in contradiction with the anisotropies detected in the microwave background by the satellite COBE.

Alternatively, cold dark matter (CDM) particles are created and have low velocity dispersions now and would have become trapped in baryonic gravitational potentials of galactic size when they decoupled. Clusters and superclusters would thus form later in the life of the Universe due to the mutual gravitational attraction of the galaxies. This bottom-up scenario is known as hierarchical clustering. Computer simulations of galaxy clustering, the observed galaxy correlation function, and recent X-ray observations of cluster evolution support the concept of cold dark matter. While consistent with the COBE observations, the CDM model needs further refinements to produce the observed large-scale structure of the Universe from the original spectrum of microwave-background fluctuations.

Candidates for cold dark matter are exotic elementary particles, including closed cosmic strings, WIMPs, axions, and supersymmetry particles, such as neutralinos, photinos, and gravitinos. Hot dark matter candidates include neutrinos with nonzero but small mass.

See also galaxies, formation and evolution.

Collins Dictionary of Astronomy © Market House Books Ltd, 2006

dark matter

[′därk ‚mad·ər]

(astronomy)

Matter that is postulated to exist to explain the rotational motion of the Milky Way Galaxy and other galaxies, to explain the motions of galaxies in clusters, and, in certain cosmological theories, to achieve the critical density of matter in the universe that is just sufficient to close the universe. Also known as missing mass.

McGraw-Hill Dictionary of Scientific & Technical Terms, 6E, Copyright © 2003 by The McGraw-Hill Companies, Inc.