Good example of an unseen phenomena deduced by the application of the scientific method, Troubleshooter!
Physicists and astronomers had to come up with an explanation for the difference between the observed luminousity of a region of space versus the gravitational field generated in that same region. Going by luminousity alone, we shouldn't observe the configurations of galaxies and stars that we actually do. The ratio of luminosity to an object's gravitational field is such that these clusters should be flying apart - not staying together. About 50 years ago a guy named Fritz Zwicky finally figured out that galaxies consist predominantly of matter in some nonluminous form -"dark matter."
There are several theories about the nature of the particles which make up this dark matter. Cosmologists look to the conditions which existed in the early universe for possible models. Likely relics of the early universe are species of stable, weakly interacting particles. One example is the neutrino, if it possesses a small mass. Normally, the neutrino is assumed to be practically massless, but a finite mass is not implausible.
Another possibility is a very massive particle (more mass than a proton!). These particles have been given the acronym WIMP: for weakly interacting, massive particle. Exotic WIMPs such as the photino have been theorized to exist in enough quantity to close the universe. The problem is that there is no guarantee that these particles DO exist. Disregarding this uncertainty, the big bang theory predicts their density today, if they do exist and are stable over the age of the universe. (and you wonder why I choose to live in a black box!
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Anyhow, I could go on, but I'm starting to detect that glazed look among the members of the audience and I have a bad cold which needs to be taken to bed with a bottle of brandy.
