Universe expansion

[rov1200]

If the universe is expanding and the galaxies within the universe are not does that mean the dark matter is being stretched to fill the void?

[jake-the-peg]

Well not really.

Dark matter has been known about for at least 20-30 years. Basically the matter we can see is only a small fraction of the matter needed to generate the gravitational force that we observe.

So we knoe there's a lot of stuff we can't see which is the "dark matter".

One possibility touted around for a few years was the possibility that neutrinos (thought masless) have a really really tiny mass. These particles are given off every time a photon is created in a star so you can see there's a lot of them! - We now know that they do have a mass although at the moment it's smaller than we can measure.

Another possibility is WIMPS or weakly interacting massive particles kind of like neutrinos but nobody's ever discoverred one. MACHOs are the more proseic possibility (Massive astrophisical compact halo objects) burnt out stars, black holes stuff like that.

So it's not so much that the dark matter is being stretched as that it may be becoming less densly populated like gas in an expanding piston. But that depends on where there is a set amount or not

[Archburner]

This is all supposition.In another 20 yrs time, some other bright spark will come up with another load of meaningless words and we'd still be none the wiser.

[rov1200]

If there is a set amount of dark matter in the universe and because of the expansion becomes less dense does that mean the galaxies will have nothing to keep them apart and over time will float into one huge mess?

[jake-the-peg]

Contrary to Archburner's assertion dark matter is not supposition but the result of some pretty fundamental and simple physics applied to some very painstaking observation. Add a little accountancy and it's easy to see that there's something left over.

The particular nature of the dark matter is supposition which is why it's called "dark".

Yes the actual fate of the universe has over the years had an awful lot of different theories. The one you describe is often known as the Heat Death one where everything fades apart and cools down to nothing.

The Big Crunch is a model where the universe falls in on itself to the Crunch described.

Since we've recently discoverred that the universe's expansion is accelerating, the crunch is somewhat out of favour so you're probably right