I am certainly no expert, but my interpretation is that it is matter that we cannot see yet has enough of a mass to exert gravity upon its neighbors.
Alot of mumbo-jumbo comes after that, but that is what I picked up. It seems that the Universe itself is sooooooooooo big that if you take all of the stars and planets and asteroids and combine their weight (their mass) it doesn't equal the total amount of anticipated mass of the universe. So the people that messed this up decided to correct their own mistake by saying that the entire universe is made up of this heavy invisible stuff we are calling dark matter. So that balances their equation for the ratio between the mass of all matter based on the size of the universe.
Without dark matter exerting its own gravity, then galaxy's would start flying apart from each other much more quickly than is happening now. So dark matter is sort of the invisible glue that is holding the universe together as it is expanding. Just think that this would always happen but dark matter is the throttle sort of, controling how fast this expansion is.
I do not think an object such as a dark matter asteroid (from Voyager) would exist. Dark matter is made up of molecules that do not interact with normal matter. The only evidence of dark matter is the image of gas being "bent" as it travels between two nearby galaxies. The "bending" is supposed to show that dark matter is distorting the light (the image we see) as these two massive galaxies approach each other. The way it is now, that kind of dark matter would never clump together to form an interstellar object. Although I did find the concept facinating about the dark matter asteroid. However if an asteroid could be created then are their dark matter people?
Again I must stress that I am not an expert on this study. Just a passionate fan of science. Their are other much more intelligent people on this topic that should chime in.
Dark matter is fun but befuddling. I am not sure if I can 100 percent believe in it, but I do know we don't have enough hard evidence, so to make conclusions about the universe is premature. You will hear different opinions about this as other posters log in.