Hubble makes 3D dark matter map

I must admit that I am very confused by this so-called dark matter. If it exerts gravitational pull on ordinary matter then surely it must have mass, and if it has mass then surely the light from distant galaxies would light it up like the sun reflecting from our moon? Also if it attracts ordinary matter then there must be dark matter on or within our planet and our sun? Another enigma that I cannot figure out is, if it neither reflects or gives out light, then how do we know that it is there, and how do we know that it exerts a gravitational pull on ordinary matter? If we are to beleive what you and your collegues say, then all matter must surely clump together one day in the future. Will this lead to another big bang, and if so then the universe has always existed, ie. a chain reaction that never ends. I would realy like a reply to these questions.

I also am very intrigued by the subject. Unfortunately, I am not at all knowledgeable on the sbuject and am unable to contribute any useful information. However. I would like to see the responses to your questions.

Thank you joeconed, as yet yours is the only response so far. I take this as no one really knows the answers.

Check out this thread.

http://cr4.globalspec.com/thread/2580/Dark-Matter-Energy

Very good questions, Scapolie!

First, about mass and reflection. You're right - dark matter must have mass. But not all mass reflects. Even with "ordinary" matter, you have transparent things like air, and black things like soot. Dark matter seems to be fundamentally different than ordinary matter; one of the differences is that it doesn't seem to reflect AT ALL.

Next, about attraction: yes, there should be dark matter in planets and stars. Ordinary matter seems to gather where there's a lot of dark matter, so there should be lots of it all around us. We just don't have a way to identify it at present.

Third, how do we know it's there? By the effect it has on ordinary matter. Astronomers can measure the rotation of a galaxy and calculate how much matter is in that galaxy. They then count up all the stars, dust clouds, gasses, and other ordinary matter in the galaxy, and it falls way short of the calculated amount! The bigger the galaxy, the bigger the discrepancy. When you look at the motions of groups of galaxies, the discrepancy gets really ridiculous. According to current measurements, there must be WAY more of this invisible Dark Matter than ordinary matter.

Last, you ask whether the universe will collapse and "re-bang". Cool idea! But current measurements indicate that the expansion of the Universe more than compensates for the pull of gravity, like a rocket moving too fast to fall back to earth. So a Universal collapse probably can't happen.

The important thing here is that all of this is theory. Captain Kirk never collected any Dark Matter to study! Look back over this little essay and notice that it's all about what we can measure so far, what we can calculate at present, and what we think we can assume. That's the exciting part about science - it's always open to change! When I talk to school kids, I always tell them: "Half of the stuff I'm telling you is probably wrong. We just don't know which half! That's YOUR job, as future scientists."

Thanks for the great questions!

I better get back to my web design stuff now, the boss might appreciate it.

What I read in the mid-nineties was that there were two types of dark matter. The first was visible in its gravitational effects. The rotation of the galaxies implies that there is an evenly distributed mass within them that we can't see. Perhaps it is dust too thinly distributed to interfere with detectable radiation.

The other kind of dark matter was described as completely undetectable, even in gravitational effects, but is required by the math applied to the big bang.