Dark matter is the term astronomers use to describe material in the Universe that is non-luminous—that is, material that does not emit or reflect light and that is therefore invisible. Everything seen when looking through a telescope is visible because it is either emitting or reflecting light; stars, nebulae, and galaxies are examples of luminous objects. However, luminous matter appears to make up only a small fraction of all the matter in the Universe, perhaps only a few percent. The rest of the matter is cold, dark, and hidden from direct view.
Because dark matter is invisible, it can only be detected through indirect means, primarily by analyzing its effect on visible material. Although dark matter does not shine, it still exerts a gravitational force on the matter around it. For example, it is possible to measure the velocities of many stars in our galaxy and in other galaxies. The measured velocities do not agree, in general, with those calculated on the assumption that the visible material of the galaxies (i.e., their stars and clouds of glowing gas) constitute all or even most of their mass. Additional, unseen mass, therefore, must exist in the vicinity of the galaxies, tugging on their stars. Such data seem to indicate the presence of massive "halos" of dark matter surrounding the galaxies that would account for most of their mass. Recent observations of the shapes formed by galaxies clumping together throughout the Universe have confirmed that dark matter does not pervade space uniformly or form structures independently of the galaxies, but is concentrated around the galaxies.
The identity of the Universe's dark matter remains a subject of research and dispute among physicists. A number of possibilities have been proposed: (1) Astronomers hold that supermassive black holes exist at the centers of most galaxies, contributing several hundred million or even on the order of a billion solar masses to each galaxy. (One solar mass is a quantity of matter equal to the mass of the Sun.) In 2001, observations of xray bursts from the center of our galaxy confirmed the presence of a large black hole there. Such black holes supply invisible mass and count as "dark matter." (2) Multitudes of non-luminous brown dwarfs or machos (massive compact halo objects)—dim blobs of gas not massive enough to initiate fusion reactions at their centers and thereby become stars—may orbit each galaxy. Such objects have been detected using gravitational lensing, but not in sufficient numbers to account for the amount of dark matter that is believed to exist. (3) The subatomic particles known as neutrinos, which pervade the universe in very great numbers, were shown in 1998 to have a small mass, ending a decades-long dispute among physicists about whether they are massless. It had been thought that neutrinos, if they have mass, might account for the Universe's dark matter; however, calculations now show that each neutrino's mass is so small that neutrinos can account for at most a fifth of the dark matter in the Universe. (4) Particles of some unknown kind, generically termed wimps (weakly-interacting massive particles), may permeate the space around the galaxies, held together in clouds by gravity.
Dark matter, which may turn out to be a combination of such factors, has long been thought to play a crucial role in determining the fate of the Universe. The most widely accepted theory regarding the origin and evolution of the universe is the big bang theory, which provides an elegant explanation for the well-documented expansion of the universe. One question is whether the universe will expand forever, propelled by the force of the big bang, or eventually stop expanding and begin to contract under its own gravity, much as a ball thrown into the air eventually turns around and descends. The deciding factor is the amount of mass in the universe: the more mass, the more overall gravity. There is a critical mass threshold above which the universe will eventually turn around and begin to contract (a "closed" universe). Below this threshold the expansion will continue forever (an "open" universe). It turns out that the luminous material currently observed throughout the Universe does not amount to nearly enough mass to halt the expansion. But what if there is a huge quantity of unseen mass out there, invisible but with a profound gravitational effect on the Universe? Dark matter, it was long thought, might supply the "missing gravity" necessary to halt the Universe's expansion.
Debate over this question persisted for decades, but has probably been resolved by observations made during the last five years that indicate that the expansion of the Universe, far from slowing down, is accelerating. If this result is confirmed, then the fate of the Universe is at last definitely known: it will expand forever, becoming darker, colder, and more diffuse.
To account for the observed acceleration, physicists have postulated a "dark energy," still mysterious in origin, that pervades the Universe and actually helps to push things apart rather than keep them together. Since energy (even "dark energy") and matter are interchangeable, some of the Universe's dark matter may thus turn out to be not matter at all, but energy.
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