How to observe the dark matter of our Universe remains a primary puzzle of astronomy. Many searches over the radiative spectrum from radio to Xrays have uncovered much of great interest, but not enough to close the Universe. Many laboratory experiments have searched directly for exotic weakly interacting massive particles (WIMPS) streaming through space, but to no avail. Gravitational lensing of more distant sources reveals dark matter in clusters along the line of sight; its amount and detailed location are quite model dependent. So far, the main direct evidence for dark matter comes from the gravitational motions of stars within galaxies and galaxies within clusters. All of it adds up to only Ω0 ≲ 0.3. Its nature, form, and distribution still are unknown.

Close agreement between the form of the GQED and the observed spatial and velocity distributions of galaxies suggests methods for constraining dark matter. It is relatively easy to start with the cosmological many-body model and formulate dark matter variations around it. These variations should not destroy the observed agreement unduly, nor attract epicycles.

As an illustration (Fang and Saslaw, 1999), consider the peculiar velocity distribution function f(v). This is especially sensitive to local dark matter. In the simplest case, the dark matter is closely associated with each galaxy (e.g., in its halo) as it clusters, and (29.4) describes the velocities. This is consistent with observations.

In hierarchical models of galaxy production, many protogalaxies merge at high redshifts. Each merger results in a spectacular wreck, which gradually restructures itself into a more unified system. Collisions engender vast conflagrations of stars as unstable gas clouds collapse and ignite thermonuclear fires. This process repeats and repeats until galaxies form as we know them today.

We see many galaxies still merging at present. In hierarchical models these late mergers are all that remain of earlier more active combining, or they result from encounters in recent dense groups. The long history of merging changes the observed galaxy distribution by destroying the conservation of galaxy numbers and by modifying the luminosity function. The first of these is easier to model; the second, at present, is really a guess. Galaxy luminosities depend on their unknown stellar initial mass functions, their subsequent star formation by merging or other violent activity, their nonthermal radiation, their production and distribution of dust, and their stellar evolution. Of these factors, only stellar evolution is reasonably well understood. On the other hand, number nonconservation depends on galaxy velocities, densities, and collision cross sections. These too must be modeled, but they seem more straightforward.

By describing the evolution of both the luminosity and the spatial distribution functions with one common formalism, we can use their mutual self-consistency to help constrain free parameters.

In the winter of 1938 I wrote a short, scientifically fantastic story (not a science fiction story) in which I tried to explain to the layman the basic ideas of the theory of curvature of space and the expanding universe. I decided to do this by exaggerating the actually existing relativistic phenomena to such an extent that they could easily be observed by the hero of the story, C. G. H.* Tompkins, a bank clerk interested in modern science.

I sent the manuscript to Harper's Magazine and, like all beginning authors, got it back with a rejection slip. The other half-a-dozen magazines which I tried followed suit. So I put the manuscript in a drawer of my desk and forgot about it. During the summer of the same year, I attended the International Conference of Theoretical Physics, organized by the League of Nations in Warsaw. I was chatting over a glass of excellent Polish miod with my old friend Sir Charles Darwin, the grandson of Charles (The Origin of Species) Darwin, and the conversation turned to the popularization of science. I told Darwin about the bad luck I had had along this line, and he said: ‘Look, Gamow, when you get back to the United States dig up your manuscript and send it to Dr C. P. Snow, who is the editor of a popular scientific magazine Discovery published by the Cambridge University Press.’