How dominant is the vacuum?

Abstract

It would be hard to find a cosmologist today who does not believe that the vast bulk of the Universe (ninety-five percent or more) is hidden from our eyes. We review the evidence for this remarkable consensus, and for the latest proposal, that the mysterious dark matter consists of as many as {\em four separate ingredients}: baryons, massive neutrinos, ``exotic'' dark matter particles, and vacuum energy, also known as the cosmological constant (Lambda). Of these, only baryons fit within standard theoretical physics; the others, if their existence is confirmed, will mean rewriting textbooks. New experimental evidence has recently appeared for and against all four components, so that the subject is in a state of turmoil and excitement. The past three years in particular have seen the fourth (vacuum) component come into new prominence, largely at the expense of the third (exotic dark matter). We conclude our review by exploring the possibility that the energy density of the vacuum is in fact so dominant as to leave little room for significant amounts of exotic dark matter.