Dark Matter |
(Think about these carefully before you consult the answers below)
QUESTIONS
1) If there is all this dark material out there in the Milky Way that we cannot see, what is it, anyway? <answer>
2) Could the "dark matter" be antimatter? <answer>
3) Who cares if there is "dark matter" or not? What difference would it make?<answer>
4) OK, so it's dark. Does that mean we cannot see it at all? <answer>
5) Where did the "dark matter" come from?<answer>
ANSWERS
1) Just as the name implies, it is any "dark" matter out there (meaning any matter we cannot directly observe or otherwise account for). Some dark matter is in the form of planets orbiting stars. Since planets do not shine by their own light, they are effectively "dark" and cannot be seen from Earth (although a few have been observed in recent years, astronomers expect that there are many more).
Small, low luminosity stars such as red dwarfs may account for some dark matter because although they shine, most are too faint to be seen from Earth. Other dark matter is in the form of brown dwarfs, which are wannabe stars that, while bigger than planets, didn't quite make to stardom. They give off heat mostly from internal gravitational compression, but they do not glow in visible light and do not undergo fusion. Predicted a few decades ago, brown dwarfs are so faint that they have only recently been observed.
Other dark matter may exist as unseen clouds of gas and dust, as well MACHOS ("Massive Compact Halo Objects," page 497). The latter objects cannot be directly seen, but have been detected by the gravitational effects they have on light from distant galaxies passing near them to produce a "gravitational lens" effect. The MACHOS may be low luminosity stars, brown dwarfs or some other as yet undescribed objects.
Some have suggested that dark matter is composed of exotic particles called WIMPS (Weakly Interacting Massive Particles). WIMPS may be any of a variety of particles that operate only through the weak nuclear force and gravity. One possible candidate WIMP are the neutrinos that pervade the Universe in staggering numbers, although the question of their mass is still not totally resolved. If they do have mass, then the enormous numbers of them may account for a significant fraction of the dark matter.
2) This is an interesting question, and there may be some evidence for matter-antimatter reactions in the core of the Milky Way and perhaps other galaxies. However, there is no good reason to believe the dark matter of the Milky Way is antimatter. First off, it would likely transform into energy as soon as it encountered any normal matter. Secondly, as best as astronomers and physicists can tell, not much antimatter exists in the Universe today. (However, it is difficult to detect, and while there is no real reason to believe so, it is at least possible that some stars or even galaxies are composed of antimatter). And third, antimatter looks pretty much like normal matter, giving it no more reason to be dark than any other form of matter. All in all, the idea seems unlikely.
3) The question of the existence of dark matter is a cosmological and even philosophical one that has no direct bearing on our everyday lives -- as far as we know. But it is like the question, "Of what value is a newborn baby?" While neither a newborn nor the answer to our question about dark matter can produce any valuable results today, who knows what the future may bring. We may someday find that the dark matter is really an immense armada of cloaked Romulan Birds of Prey, or we may find out the ultimate fate of the Universe. In either case it would be very interesting information to know!
4) "Dark" here is meant both literally and metaphorically. While it may be truly dark in visible wavelengths (such as some nebulosities and brown dwarfs), it also can apply to objects that are simply too small (such as meteoroids) or too dim (such as some red dwarfs) to be seen. The bottom line is that it goes undetected, not merely just unseen.
5) Presumably, dark matter came from the same source as all matter, the Big Bang and subsequent transformations within stars. It may be normal ("baryonic" or composed of protons and neutrons) matter that we just haven't directly detected (as as dust, individual atoms and molecules, planets, brown dwarfs and even red dwarf stars); or it may be the more exotic (non-baryonic) varieties that are not just dark, but can't be seen because they don't operate under electromagnetic forces (such as the WIMPS).
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