The words "dark matter" are a shorthand for an enormous range of evidence indicating (a) that various astronomical mass-to-light (M/L) ratios are larger than can be accounted for by visible stars and gas at any temperature and (b) that M/L systematically increases as one measures it on larger and larger distance scales. The evidence is reviewed historically and attention given to the range of possible gravitating substances (collectively "dark matter candidates") that might make up all or part

more »

... f the stuff. A handful are currently taken seriously, but the total inventory is at least several dozen. Dark energy comes at the end, but is also to be taken seriously. 1.