well frankly kids, no one KNOWS what dark matter IS. we do know however that it is THERE!!!!
By careful study of the dynamics of galaxies and the motions of stars astronomers can deduce the presence of mass by the gravitational effects it has on luminous matter(like stars).

there are some very exotic and very mundane proposals (hypotheses) that are promulgated. but there is definitely no clear consensus. Everything from some weird attribute of STRING theory to mundane black dwarf stars...or rogue planets, as well as what you mentioned.

this is an interesting and open question that is getting a lot of interest from researchers.

The truth is, nobody knows what most of the dark matter is.Knowing Kepler's Laws and the velocities of stars in a galaxy or galaxies in a cluster, it is easy to figure out the mass necessary to keep the galaxy or cluster from flying apart. But when astronomers add up the mass of everything they can see with their telescopes, the mass of the Universe comes out to only 1-2% of the mass revealed by the law of gravity. The rest is the mysterious, invisible Dark Matter, which makes it's presence known only through it's gravitational pull. Surely, some of the dark matter is just ordinary atoms in the form of dead or stillborn stars, thin gas, and black holes (Black holes aren't made of atoms, but they grow by feasting on atoms, so for the purposes of this argument it's as if they were made of atoms). Now, we know that all the atomic dark matter could not possibly make more than about 5% of the mass of the Universe. About 3 minutes after the Big Bang, the Universe was a soup of protons, neutrons, and electrons that would eventually become today's atoms. Some of the protons and neutrons stuck together to form helium, deuterium, and small amounts of other light atoms that have been around since the beginning of the Universe. Heavier atoms, like the ones we are made of, were forged in dying stars and flung into space when those stars exploded. We have a good idea of the number of protons and neutrons in the early universe because their density affects the abundance of light elements. If any more than about 5% of the mass of the Universe started out as protons and neutrons, then it would have been much easier for these particles to meet in the early Universe and make light elements, and we should see more of these elements today. So, no more than about 5% of the Universe is "ordinary" matter (stars, planets, gas, dust, black holes, etc.).
Some of the other 95% probably comes from ghostly particles called neutrinos. They interact with ordinary matter incredibly weakly: Lead is excellent at blocking all other forms of radiation, but you would need a 10-trillion-mile lead shield for a good chance of stopping a neutrino. Billions of neutrinos fly through you each second, but you never know the difference. So how do we even know they exist? Because when they are created in certain nuclear reactions, energy and momentum are not conserved unless you assume there is a nearly massless invisible particle which carries away some energy and momentum. Also, once in a great while, neutrinos do interact with matter in a detectable way. For a long time they were thought to be massless, but recent experiments indicate that some types of neutrinos have a very tiny mass. If this is so, then neutrinos could make up much of the dark-matter mass through sheer numbers. In the early Universe, there should have been about 100 million neutrinos created for every hydrogen atom (I'm not so sure about this number, but I know it's big). However, there is one problem with neutrinos as dark matter: With such small masses, they would have been set flying too fast to clump together gravitationally. We know there are very large clumps of matter in the universe (galactic superclusters) and computer simulations indicate that the dark matter needs to be slow-moving in order to form these dense clumps. Dense clumps of dark matter are needed to explain how ordinary, visible matter remains gravitationally trapped in galaxy clusters, as explained above. Neutrinos surely make up some of the dark matter, but the rest remains unidentified.
So, the types of matter we know about (protons, neutrons, electrons, and neutrinos) cannot solve the dark matter puzzle. Most scientists think there must be undiscovered types of heavy, slow-moving, weakly interacting particles making up the bulk of the Universe. There are some stranger theories too- such as parallel universes interacting with ours only through gravity and creating the illusion of matter in this universe, or Newton's law of gravity not working at large distances. I suspect you will see the mystery solved within your lifetimes, but only time will tell.

Here you have me. I don't know what dark matter is either. I can briefly describe what the evidence for it is... Galaxies are know to be very large groups of stars, usually rotating about some axis. The size is so large that the rotation period is tens or even hundreds of millions of years. If there were no dark matter, each star would orbit the center of mass of the galaxy more or less independent of the others -- this is because the distances between the stars are so vast. In such a model, the stars close to the center of rotation should be traveling substantially faster than those at the rim...
Further, measured velocities of stars in many galaxies are well beyond the escape velocity of the galaxy-- if the galaxy mass is mostly in stars.
Measurements of velocity distributions (made by spectroscopic analysis) for typical galaxies show that the galaxy rotates as if it were embedded in a "soup" of uniform density -- i.e. that there is something with a mass comparable to the galaxy which doesn't show up on the photos -- i.e. dark matter. It has also recently been found that a large number of galaxies have black holes in their centers (or close)which does add substantial mass to the galaxy, but only makes the velocity distribution worse.

Possible dark matter candidates are:
1. Machos (sub-stellar mass dark bodies of ordinary matter -- distributed fairly uniformly in the galaxy halo.
2. Large numbers of quantum black holes -- (largely discounted)
3. Some new massive non-interacting particle -- note that although the guessed mass of neutrinos is very small, there are lots and lots of neutrinos, such particles could make up substantial mass in a galaxy.
4. Prof. Alfevin suggests that there are long distance effects of electromagnetic factors.. i.e. maybe gravity isn't the only force acting on such large scales.

There are ongoing searches for both 1 and 3 ... jury is still out.

Well, no one really knows what dark matter is for sure, but we know what it probably ISN'T.

Let's start by seeing how we know it is there: the first clue was in the motions of stars in galaxies. If the matter we can see is all the matter that is there, then we can predict how the stars should move. But they don't move that way! And it looks just like there is a big cloud of matter surrounding each galaxy that we can't see. We can also detect that there is extra matter around galaxies by measuring how galaxies bend light rays from other galaxies (and quasars) from behind them.

One idea is that dark matter is made of small planets or "brown dwarf" stars that never quite started shining, but astronomers have actually been able to look for those. They don't find enough to account for all the dark matter. So probably the most popular idea is that it is an unusual kind of particle, like an electron, except that it can only be seen by gravity (electrons have electricity and magnetism, too).

Here's the kicker: astronomers also look at how the big bang created elements (like helium), and the measurements seem to show that the dark matter cannot be "ordinary" matter, like protons and neutrons. So try to find other ideas for what dark matter could be! This is a very busy research topic.