Iran- You're right that most of the energy in the universe is in some unknown forms which we can't see directly. So how do we know it's there?
Let's start with the so called ídark matterí. Thereís about five times as much of it (by mass) as there is of ordinary matter. Of course we can only see some of the ordinary matter, mostly the part which glows brightly because itís in stars. But at least we can see some of it. The dark matter shows up because it has gravitational effects on ordinary matter. It helps hold galaxies together, and it helps make galaxies clump. It also has affected the past expansion of the universe. We can see all these effects by looking either at light from stars or by looking at light left over from the big bang. There are various ways to know that the stuff having these gravitational effects canít be ordinary matter. One way is by considering the equilibrium between different types of nuclei back in the early minutes of the big bang. It was kind of like a chemical equilibrium. From the relative amounts of hydrogen, helium, deuterium, etc, you can tell pretty well how much ordinary matter was around, and then calculate how much is still around.
People have various guesses about what that dark matter might be, but none are either confirmed or theoretically compelling.
Perhaps even more mysterious, thereís about twice as much 'dark energy' as there is matter of any type. Dark energy has a peculiar property. Its density doesnít change (or changes only a little) as the universe expands. It turns out that that makes gravity push it apart instead of pull it together, the opposite of the effect on matter whose mass rather than density is fixed. That makes the expansion of the universe accelerate. There are various pieces of evidence to confirm that, including measurements of how the ripples in the light left over from the Big Bang have spread out.
No one piece of evidence for any of this is compelling all by itself. Itís the combination of many lines of evidence, fit together in a coherent theoretical framework, that makes the story believable.
One of the best sources for good articles on this topic is Scientific American.
(republished on 07/19/06)