Ok I grasp (barely) the balloon concept in the "Outside" thread. Where does dark matter fit into this? Any good concepts on what it is?

Dark matter is matter that cannot be seen, but which can be detected by its gravitational effect on other bodies. Perhaps up to 90 percent of all matter in the Universe may be dark matter. Some of it is simply planets around distant stars that cannot be seen because they give off no light except the reflected light from the star they orbit, and this light is too faint to be detected from such great distances. Black Holes may also be considered dark matter, because no light can escape from its event horizon.
But this still is just a small portion of dark matter, and some say that the bulk of it may be neutrinos, one of the fundamental particles of the universe, that have a very very very tiny mass, but since there are zillions upon zillions of these uncharged particles, a lot of a little could add up to a lot.
By and large, dark matter is still a mystery. Recent discovery that our Universe is expanding at an accelerating rate , seems to have confirmed the existence of a dark energy, or vacuum energy, which acts as sort of an anti-gravity that may cause the universe to expand forever, overcoming the gravitational pull of the universe itself, although I wouldn't count on that yet, Gravity may be the greatest force of all. Scientists are trying to find a link between this dark energy and dark matter.
The most spectacular theory of all, however, is that dark matter may be matter from a parallel universe...one outside of our 4-D 'balloon' space-time 'bubble' universe.... that can never be seen (because light and x-rays and all forms of electromagnetic radiation cannot ever escape from nor enter our bubble), but again ehich can be detected by its gravitational influence, because gravity waves, unlike electromagnetic waves, may be able to traverse through the higher order dimensions of space time. I like that theory the best of all.

Thank you Gerry. I have heard the 90% figure before. I would like to see the state of the sciences in 30 years or so. Will we recognize it?

Knowledge of the Universe is increasing daily at a rapid pace, largely due to the Hubble Telescope, tremendous technological advances, and the genius of the theoretical physicists. A gravity wave detector (LIGO) may be functional by the end of the year, searching for gravity waves hundreds of thousands light years away that are so weak that even black hole collisions that tear space time apart and send such waves toward us at the speed of light, may reach us undetected. However, it is expected by some that the so called M-Theory, which may be the "Theory of Everything (TOE)" and which is the hottest topic today amongst the theorists, may be around for another 50 years or so before we have a better grasp of the answers, and for certain, we won't have all of them.