Scientific principles and laws do not lie on the surface of nature. They are hidden, and must be wrested from nature by an active and elaborate technique of inquiry. ~John Dewey
Here's a little science fair project for you. Add up the mass of the observable universe; be sure to include all the interstellar gas, every little hydrogen atom, every detectable gamma ray and photon. Be sure to include any energy running around loose, because, as Einstein told us, energy and mass are equivalent. Okay, once you've done that, see if that amount of mass accounts for all the observable gravitational effects that you detected while you were toting up the masses.
All right, you already know this is a trick question, because all the observable mass and energy don't account for all the observed indirect effects of mass and energy seen in the universe. What may boggle your mind is the current thinking about just how little of the universe we can actually see and (currently) directly measure. Visible mass accounts for about 5% of what should be there.
Talk about the proverbial drop in the bucket. But it gets even better (or worse, depending on how you feel about things you can't see). Dark matter, some sort of exotic stuff we can't see or detect, only makes up another 25% of the total. It turns out that something even more exotic called dark energy makes up the remaining 70%. So 95% of the universe is made out of stuff we can't directly detect, see, feel, touch, or taste ... at least, so far.
Now, missing most of the matter in the universe is highly annoying. It's also detrimental to developing working theories on the formation of planets, stars, galaxies, even the universe itself. This is not a state of affairs that scientists care for. So, they keep looking.
So far, there are more hints as to the existence and properties of dark matter than dark energy. But, given the mass-energy equivalence, if you can determine the nature of one, you should have a leg up on exposing the other. One candidate for the missing matter is the neutrino.
For years, the neutrino was thought to be massless or very nearly so. In recent years, though, it's been determined that there are various types of neutrinos and they have varying masses, still tiny but definite. And, there are huge quantities of neutrinos floating around out there. Recently, scientists have determined that supernovae release immense quantities of neutrinos, based on theory and based on observations of supernova 1987-A, the closest supernova to occur in modern times, thus the best observed as well. When 1987-A was seen to explode, a wave of neutrinos were picked up on detectors on Earth.
There have been innumerable supernovae over the history of the universe, so there ought to be a shimmering sea of neutrinos about us. The trouble is the nature of these particles makes them very difficult to detect. Neutrino detectors have moved beyond the infancy stage, but they still lack the ultimate sensitivity needed to find these elusive particles. A new generation of detectors is poised to join the search, which may give us a chance to detect these ancient neutrinos.
Supernovae also gave us the first hint of dark energy. As stated in the same article, it was found that light from type !a supernovae was dimmer than expected, which scientists theorized was due to dilution by dark energy, a repulsive force of an unknown nature (which didn't stop the media from calling it “anti-gravity). Perhaps there's a relationship between the neutrinos and dark energy.
Recently, another interaction between supernovae and dark matter was inferred from computer simulations. According to the simulations, dark matter density should be concentrated in center of galaxies, but it seems to be be much more diffuse, based on actual observations. Among other ideas for why this should be is that supernovae shock waves may be dispersing the exotic material. The thing to keep in mind here is that computer simulations depend heavily on the assumptions built into them. While many of them are excellent for making predictions, when their results don't match reality, it's difficult to tell whether we don't understand nature or we built too many assumptions into the simulations.
This isn't a complaint; it's a statement of reality, which the scientists recognize. The benefit to the simulations is that they give you some idea of what your current theories would do if you could actually watch the early universe in action. Good scientific methods question both the theories and the simulations, refining both until everything comes together. Then, of course, someone builds a better computer which show new problems with the simulations, and you're back to the drawing board.
This is what scientists call “fun.”
Then there's a group that says maybe the whole dark matter thing is a bogus issue. Perhaps our theories of gravity are out of whack with reality. Challenging Newton and Einstein without something very firm to replace them is strong stuff, so much as many astronomers and physicists dislike the dark stuff, it seems to beat the alternative.
One group has used the Chandra X-Ray Observatory to gather information that seems to keep gravity in tact and come close to actually showing us dark matter. They didn't see it directly, but the scientists' data shows gravitational effects consistent with some exotic material being in the mix. Many stories are touting this as absolute proof of the existence of dark matter. Doug Clowe goes so far as to say, "We've closed this loophole about gravity...”
Well, perhaps, but I'm not so sure about that. We're still invoking something we can't see and can't define to explain an observed effect. The accumulation of data indicates something is interacting with visible matter, but until we know what it is, I don't think any “loopholes” are closed. If history teaches us anything, it's that we can come up with ingenious explanations for observed events that can be utterly wrong. Ultimately, the challenge is to more fully characterize what dark matter is, rather than just what it does.
Just don't call the IAU to help with the definition. Dark matter might disappear altogether.
(No, I didn't forget about the Pluto formerly known as a planet; tune in Saturday.)