"Regarding the study of the universe on scales of less than... 108 lt-yr... [T]he "top-down"... or "pancake" theory of Zel'dovich et al... [predicts that] pancakes of gas comparable to a were formed first... then galaxies... formed by fragmentation... [into] high-density regions... along lines, filaments and points... The problem of the missing mass, or ... and... of the agreement with the observed level of isotropy of the blackbody radiation... are explained in this top-down model by massive neutrinos... which formed early condensations in the homogeneous plasma... [were] originally moving with relativistic speeds... [and] are usually called Hot Dark Matter. [Another] theory is the "bottom-up" scenario discussed by Lemaître and supported by... Peebles et al... [in which] galaxies were first formed by gravitational interaction... [and] only afterwards... a hierarchy of clusters formed. Some versions of [this] theory explain the missing mass problem... by... exotic particles such as "s," "s," "s," and "s," predicted by some unified field theories. This exotic form of dark matter, moving... much slower than the massive neutrinos, is... called Cold Dark Matter. Mixed models, hot plus cold dark matter, have also been proposed..."