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How is dark matter different from ordinary (visible) matter?
1) Does dark matter consists of invisible astronomical objects like white dwarfs, black holes?
2) Has dark matter not been directly observed?
Is there a better explanation for dark matter. I could not understand completely the difference between dark matter and ordinary matter.
There is certainly one and possibly two types of dark matter to solve two dark matter problems.
The first dark matter problem is that most gravitating matter is dark. Most of the inferred mass that appears to be responsible for the observed gravitational dynamics of large scale structures like galaxies and clusters of galaxies cannot be seen (i.e. is dark). That is, we can't detect it through visible light or any other electromagnetic waves.
Now it is possible that you could make up this mass out of very low mass stars, cold white dwarfs, planets, black holes, lost golf balls or indeed any other form of cold, non-luminous matter. Such objects (bar the golf balls perhaps) certainly do exist, are known as baryonic dark matter (because they are made of "normal" protons and neutrons - baryons), but there is unlikely to be enough of this stuff to explain the observations.
The second problem is that most of the dark matter must be non-baryonic. This is deduced from the lack of sufficient plausible baryonic dark matter to explain the dynamics of galaxies and clusters, but also from estimates of the primordial abundances of helium, deuterium and lithium. Big bang nucleosynthesis depends on the density of baryonic matter compared to the universe as a whole and it looks like baryonic matter can only be responsible for a fifth of the dark matter.
Thus the bulk of the dark matter is supposed to be some non-baryonic stuff, probably in the form of particles that do not interact via the electromagnetic interaction and therefore do not emit or absorb light.
Nobody knows it for sure.
What is known:
"Dark" doesn't mean "black" here, it means here "invisible".
There are many calculations and theories, the most popular one says it may be from LSPs (least supersymmetric particles).
Read Before the Big Bang (1997) by Ernest Sternglass. The short answer is that a big piece of "dark matter" is the source of a quasar.
The "non-standard" and therefore controversial theory presented in this book says that most "dark matter" might be
“fragments of the original primeval atom [of Georges Lemaitre’s model --- please google it if you need to] ejected to large distances in the explosive ‘mini-Bang’ that had to accompany the formation of every cosmological structure [ie, every moon, planet, star, galaxy, etc.] in a Lemaitre-type model … the existence of quasars and active cores of galaxies over a wide range of distances indicated that there were apparently delayed mini-Bangs in which new galaxies were created, as Maarten Schmidt had conjectured, together with vast amounts of dust and gas ejected into space” [p.211, BTBB].
Plus: “All these results strongly suggested that some of the original fragments … from the Lemaitre [“primeval”] atom had managed to survive in the massive centers of large galaxies for very long periods … these massive electron[-positron] pair fragments were apparently ejected long after the Big Bang, as the Russian astrophysicist Novokov and the Israeli physicist Ne’eman had in fact suggested independently in the mid-1960s … their nuclei would be so massive that they would be invisible black holes, yet they could account for a dominant fraction of the total mass of the universe even today” [p.212, BTBB].
According to Sternglass’s model, these fragments of the primeval atom are “seeds of galaxies and stars”, and are spread throughout our universe, ever since the Big Bang. He says that a seed remains dormant for millions or billions of years,lurking in space, and then, after a long “count-down” process, during which the system divides in half, again + again + again, it suddenly explodes, violently. He calls this explosion a “mini-Bang”, and says that the “gamma-ray bursters” —(also called “quasars”)— which astronomers have observed since the 1970s, are in fact the “delayed mini-Bangs” which his model predicts.
