Invisible matter—one that does not emit and does not absorb light and that reveals itself by gravitation it generates—has been dubbed by astrophysicists dark, or exotic matter. It is ubiquitous, present everywhere, in individual galaxies and in galactic superclusters alike. In mass it far exceeds visible, ordinary matter. What is it really like? We cannot tell. It might be composed of some unknown elementary particles, or be in the form of small-mass black holes or hypothetical "worm-holes". This has been the subject of an English-written article by RAS Corresponding Member Igor Novikov, who is also a member of the Astrocosmic Center of the Moscow-based Lebedev Physics Institute, and the Niels Bohr International Academy (Copenhagen, Denmark). Translated into Russian by RAS Corresponding Member Viktor Abalkin, this article was carried in the Russian-language journal Earth and Universe. We follow with an English transcript.

Dark matter is one of the mysteries of present-day cosmology. The discovery and research of this phenomenon has a rather long history. For as much as eighty-five years astrophysicists have been closely involved with this subject. Today this problem has come to the fore.

Thirty and even twenty years ago astronomers maintained that the mass of dark matter was predominant in the universe and determined its dynamics and the curvature of its spacetime. Today we  know much  more. Observations within the range of anisotropy temperature measurements of the cosmic microwave background radiation (such radiation appeared immediately after the birth of the universe, and it carries important information) as well as data on the presence of helium and other light elements and on the structure of the universe—all this indicates that ordinary (baryonic)* matter is responsible for about 4 percent of the cosmic mass. So the stars, planets, gas, dust and we ourselves are composed of such visible matter, while the rest 96 percent is a "dark", exotic spectrum with about 23 percent of dark matter and about 73 percent of dark repulsive energy. We know that ordinary, visible matter produces the effect of gravitational attraction, while dark matter and energy, on the contrary, that of gravitational repulsion. In real terms dark matter (energy) predominates in the universe, though nothing is known about its physical nature so far.

Dark matter has a gravitational effect on the propagation of light from remote sources (the effect of Gravitational Lensing so-called). A significant portion of information also comes from the analysis of microwave background radiation and of the process of the structural formation of the universe from minor initial inhomo-geneities. But the dark matter gravitational force is nec-

* Baryonic elementary particles with a mass not smaller than that of protons are implicated in all basic interactions.—Ed.

Giant galactic cluster CL0025 1654, which is 4.5 bln light years away. Hubble Telescope. Photo taken on Aug. 14, 2003. NASA. Jean Paul Kneib.

Hot gas region in the universe. The shot made on April 4, 1999, by the International Space Roentgen Observatory ROSAT, NASA. R. Nemirov, Jerry T. Bonnell.

essary for the formation of galactic clusters and galaxies as such. Most cosmologists, Igor Novikov continues, postulate that dark matter is cold. Many think it is composed of particles formed in the early, hot period of the evolution of the universe, and these are still there. The list of constituent elements is rather large: they are predominantly hypothetical particles, say, axions or supersymmetrical relicts. Experiments are underway toward their direct and indirect search. Dark matter might be identified after all, but its physical nature is still obscure.

Apart from elementary particles unknown yet to physicists, there may be other objects within dark matter as well. Some are just wonderful per se—and just as important to science: say, relativistic dark bodies (primary black holes and "wormholes").

The black holes hypothesis has a long history, too. Thanks to the studies made by our physicists, Acad. Yakov Zeldovich and Igor Novikov, in 1961 and then, in 1971, by Dr. S.W. Hawking of Britain, we can infer: at the early stages of the universe (ca. 13 billion years ago) there were tiny black holes smaller than stars in mass. It is estimated that those with an initial mass below a billion tons have lost all of their energy because of quantum radiation, while the heavier ones have survived.

Spiral Galaxy M83 (NGC) in the Hydra Constellation 15 mln light years away. Photo made on May 25, 1998, in the Kitt Peak National Observatory, Alabama University, William c. Keel.

Abell 1689 cluster of galaxies two billion light years away. One of the most massive objects in the universe responsible for the Gravitational Lensing effect. Snapshot made in January, 2003. Hubble Telescope, NASA.

Is it really possible to detect black holes by available astronomical facilities if they are indeed in the universe? That is the question. To discover rather small black holes we should know their hard quantum radiation. Such kind of radiation might have been helpful in the identification of primary black holes; yet none has been discovered thus far. What we have found is this: the number of black holes with a mass about a billion tons each is not above one thousand per cubic light year. Had there been more, we could have calculated their aggregate emission. The quantum radiation of massive black holes, however, is insignificant, and therefore they could be assigned to objects within dark matter. In 1994 our astrophysicists Pavel Ivanov, Pavel Naselsky and Igor Novikov, who worked as visiting researchers in the Danish Center of Theoretical Astrophysics, pointed at this prospect. Simultaneously, a microlensing of stars by our Galaxy's halo objects was reportedly detected in the Large Magellanic Cloud. Such objects could be black holes, said one hypothesis. This discovery was yet another argument in favor of dark matter being composed of primary black holes.

We should not forget about the "wormholes" either. According to the general theory of relativity, these are greatly curvilinear spaces in the form of a tunnel and two inlets (outlets). Once matter or radiation gets in, it disperses all along the tunnel and comes out of the other hole. Or just the other way around. According to one hypothesis, these primary holes must have appeared at the start of the initial expansion of the universe. And they could persist. We might as well note that quantum evaporation (otherwise known as Hawking evaporation) has no effect on like objects, and thus they continue for cosmological time spans if not subject to certain instabilities. So part of cold dark matter might be composed of wormholes.

Thus, Dr. Novikov concludes, dark objects, or primary black holes and primary wormholes, may hold a clue to the dark matter enigma. How good (or not good) are the present theories? We'll learn once the observational results are in on cold dark matter explored above all by the space observatory Planck launched on May 14, 2009, within the framework of the European Space Agency Horizon-2000, and named for the outstanding German Physicist Max Planck ( 1858-1947).

/. Novikov. "Dark Objects and Dark Matter". "Earth and Universe" No. 5. 2009