Cosmological observations have established that the known universe started as a small patch of space filled with radiation that subsequently expanded and cooled. In the earliest stages of its evolution, the universe contained extremely energetic particles, and it is likely that some important relics that could have been produced only at these early times are likely to remain today. Indeed, astronomical and astrophysical studies may very well be a way to study some aspects of particle physics beyond the Standard Model.

Particle physics interacts with cosmology on three important topics: dark matter, structure formation, and baryogenesis and nucleosynthesis.

There is very strong evidence for a preponderance of dark matter in the universe, and there are strong arguments that it cannot be ordinary matter. This may be seen as one of the most important scientific discoveries of this century.

The amounts of various types of matter in the universe are customarily expressed as a fraction of what is called the critical density. The critical density is that value above which the universe will eventually contract and below which it will continue to expand forever. Of course the value of the actual total mass density in our universe is of great importance.

From observations, one gets important information on the dynamics of the matter in the universe. One looks at the radial dependency of rotational velocities of stars in spiral galaxies. These observations indicate that galaxies are made of visible stars in the center and are surrounded by massive halos of invisible matter. Additional observations using radio observation of the rotational velocities of neutral hydrogen in the gas clouds indicate that the halo extends way beyond the edges (defined by visible stars) of the galaxies. From these observations and also from studies of gravitationally bound clusters of galaxies, it is known that the mean mass density in the universe is at least 20% of the critical density and could be consistent with being equal to the critical density. In addition, there are indirect but compelling theoretical arguments as to why our universe actually has almost exactly the critical density; however, the question remains open.

Now, what is the composition of the matter in the universe? One can readily add up the matter in luminous mass (stars), and this is found to be less than 1% of critical density. Thus there is at least 20 times more mass in the universe that is nonluminous (i.e., "dark").

The question arises as to whether this matter is ordinary (baryonic) or possibly exotic (nonbaryonic). The amount of ordinary matter in the universe is, however, constrained by the big bang model to be between about 1% and 10% of the critical density. Thus, there is a major component of baryonic matter that cannot be accounted for in observations, and if the universe indeed has the critical density, then it is dominated by nonbaryonic dark matter. Even if not, there