What is This Stuff??? Illustration from "The Particle Adventure," http://particleadventure.org/particleadventure/frameless/dark.html

One of the basic questions in physics and astronomy is what is the dark matter. We could get out of this question if we just assumed that the law of gravity was breaking down, and that the galaxy rotation curves and all the other consequences we ascribe to dark matter were due to an incorrect gravitational force law. However, we don't believe that can be correct because the Newton/Einstein form of the law is so successful in so many situations. In fact, we have direct confirmation on scales and conditions that are not wildly different from galaxy diameters that the law works very well. This situation contrasts with the modifications Einstein made to Newton's form of the laws that only became apparent under much more extreme conditions than Newton had considered. This point is made clearly by the difficulty in confirming the bending of light predicted by Einstein. 

The problem then comes down to imagining forms of matter that could be hidden, and then seeing if we can find them. One possibility is that the dark matter is in the form of ordinary matter (baryons) that is locked up in objects that are hard to see. For example, the matter might be in planet-sized chunks that would not glow by their own light and would not be close enough to stars to reflect any significant amount of light.

One way to find such objects is to watch a dense field of stars night after night. If an unseen object passes right in front of one of the stars, the gravitational bending of the light will make the star flicker and may even make it look brighter. Here is such an event, in the center of the image. Each frame is about a week later than the preceding one..

Even though a number of search approaches have led to evidence for objects the size of massive planets, the numbers just don't seem to add up to nearly enough for the dark matter. Therefore, scientists have turned to particle physics for a solution. Some possibilities are:

• axions - very light particles with extremely weak interactions (hence very hard to detect) - dark matter would require that the Universe be filled with them at a density of about 10⁸ per cubic centimeter
• WIMPs - for weakly interacting massive particles - maybe 10⁷ times as massive as axions, so we need only 10 per cubic centimeter!

Although some physical theories predict the presence of WIMPs, basically we have to say that axions and WIMPs are both the subject of physicist's dreams rather than some well-determined solution to the dark matter puzzle. We really don't know what the Universe is made of!