I wrote one long complex answer, but was not signed in and when I was asked to sign in, my answer disappeared. I believe this must also be somewhat related to dark matter. It mysteriously swallowed my answer! Anyway, the true answer to your question is as follows. From observations here on Earth, and physical calculations, we (humans) have managed to make some pretty good assumptions as to the aggregate total of all the observable, or otherwise detectable, mass in the universe. When we plug this figure for the total mass into the equations describing the observed motion of the galaxies in our universe, we find that the mass of the detectable parts of our universe is no where near enough to make the equation balance. In order to make the equations of motion balance, we need to have a lot more mass than we have been able to surmise exists based on visible matter. Hence, we have come up with a conundrum that can not be solved unless we allow for the existence of a huge amount of other matter. Since we cannot detect this matter with our current abilities, we have called this dark matter. Its' existence balances our equations, but is not detectable with our current technology and mental abilities.

I, aside from science, venture to say it is the compressed form of a black hole.

Hopefully we will get more concrete answer once the experiments begin at the CERN particle accelerator next year.

The leading candidate particle for Dark Matter is the hypothetical WIMP particle. WIMP stands for Weakly Interacting Massive Particle, and is theorized to be extremely heavy and therefore difficult to create with a particle accelerator.

It is postulated to exist, as another said, but we don't really know exactly what it is at this point. That's part of why it was named 'dark' originally. It may be related to relatively newly discovered dark flow?: http://www.space.com/scienceastronomy/080923-dark-flows.html

Dark matter is matter we can weigh indirectly, so we know it's there, but we just can't see it. Anti matter is matter with a exact opposite charge of matter itself. The two are not the same, and I wanted to point that out. Dark matter is the "stuff" that makes up the major "bulk" of our visible universe. The problem is, we only see a little over 10% of what we estimate the universe to "weight" in matter itself....remember, not "anti-matter", which, in and of itself very small in numbers, because when it meets with matter, they annialate each other. The only reason our Universe is made up of "positive" matter, is because it outnumbers "nevative matter" very slightly, but enough to substantiale the universe as we know it from the remnants of the annialation. There are many speculations to what dark matter is. I believe we aren't looking in the right place for answers, because those answers lye in a path of thinking we haven't yet realized. We will, and I believe it will be through string theory. I have a gut instinct that dark matter is perhaps a direct result of dynamics between the 12 upper dimentions in our Universe. We just have to invent the math, (we are slowly making strides) in order to solve this puzzle competley, That's my theory, I beilieve it has to to with the ever illusive "graviton"...(theoretical still)...

dark matter is the substance covering areas in my brain either from too much drug use or areas of my brain which have not been used in a long time or ever hope this helps. let me know if you need a visual

It is energy. And since energy cannot be created or destroyed, it is there for us to feel it with our energy but have it not be seen

Dark matter is basically stuff we can't see. I mean that you can't detect it directly with current detector technology. It exists, and acts like matter because it attracts galaxies and clusters of galaxies gravitationally. And for these structures to form within the time the universe has existed, there needs to be a lot more matter present than we can observe.