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The Net Advance of Physics: The Nature of Dark Matter, by Kim Griest -- Introduction

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The dark matter problem has been around for decades, and there is

now consensus that we don't know what the most common material

in the Universe is [1]. It is ``seen" only gravitationally, and does not

seem to emit or absorb substantial electromagnetic radiation at any

known wavelength. It dominates the gravitational potential on

scales from tiny dwarf galaxies, to large spiral galaxies like

the Milky Way, to large clusters of galaxies, to the largest scales yet

explored. The universal average density of dark matter determines

the ultimate fate of the Universe, and it is clear that the amount

and nature of dark matter stands as one of the major unsolved

puzzles in science.

In this review I will first briefly recall the evidence for dark matter,

with emphasis on the dark matter in our own Galaxy. I then turn to

the dark matter candidates and how we might discover which (if

any) of them actually exist. Then, I will focus in on two of my

favorite candidates, the supersymmetric neutralino Wimp

candidate, and the baryonic Macho candidate. For the latter

candidate, I will go into some detail concerning the one particular

experiment with which I am involved, and present some results

showing, that over a broad range of masses, this candidate has been

ruled out as the primary constituent of the dark matter in our

Galaxy. In my discussion of the supersymmetric Wimp, and

especially in discussing the neutrino and axion candidates, I will be