IS THE UNIVERSE GRAVITATIONALLY DOMINATED BY NONBARYONIC DARK-MATTER

A new method for deriving a lower limit of the mean density of the universe using the maximum temperature of the X-ray-emitting hot gas in clusters of galaxies is proposed. This method is applied to the present record holder for the highest temperature among X-ray clusters, A2163. I find that the probability that the observed volume, in which complete X-ray observations are performed, contains at least one cluster with a temperature as high as the temperature of A2163 is very small for a low-density universe. Even if we take the most conservative upper limit of the baryon density of the universe derived from big bang primordial nucleosynthesis theories, the possibility that the universe is made from only baryonic matter can be rejected with more than 99.99% confidence. Applying the condition that the expectation of finding a cluster which has a temperature higher than A2163 in the observed volume is at least 1, the mean density of the universe must be more than twice the baryon density of the universe. I conclude that it is very probable that our universe is gravitationally dominated by non-baryonic dark matter.