The Milky Way - Andromeda Flaw?

Several times I've read and heard that all the galaxies are moving away from each other. The way this is occurring is that the space between the galaxies is filling in with dark matter, thereby pushing all the galaxies further and further away from each other.

Question: What's wrong with the dark matter that's supposed to be filling in the space between the Milky Way and the Andromeda galaxy? Why are we on a collision course instead of being pushed away?

Our two galaxies are close enough that the gravitational force is greater than the force causing expansion. At least I think that's what the explanation is.

I heard this dark matter is also pushing on atoms from the inside seperating them. So visually is see kind of rain drops on water, all expanding at the same time, every ripple expanding, at some point they all go towards each other.

Originally posted by OMsk3ptic
Our two galaxies are close enough that the gravitational force is greater than the force causing expansion. At least I think that's what the explanation is.

I would be happy with that if not for the fact that there are galaxies even closer to the Milky Way, but I've never heard of them being on a collision course with us.

This from universetoday.com: Closest Galaxy to the Milky Way

The Andromeda Galaxy (M31) is the closest spiral galaxy to us, and though it’s gravitationally bound to the Milky Way, it’s not the closest galaxy by far, being 2 million light years away. The Large and Small Magellanic Clouds are two irregular dwarf galaxies about 180,000 and 210,000 light-years away, respectively. They were thought to be orbiting the Milky Way, but that may not be the case.. All of these galaxies make up part of what is called the Local Group, which is a group of more than 30 galaxies that lie within 4 million light years of the Milky Way. Read more: www.universetoday.com...

reply to post by OMsk3ptic


Yeah, that's a pretty great way to sum it up!

Gravity is the weakest of all the forces, but it's felt over larger distances. The larger the mass, the more it ads to the force. While our entire universe is expanding, and filling with dark matter, galaxies are formed in groups and clusters that are gravitationally bound and don't expand in the same way as the rest of the universe. The groups may be moving away from each other, but the galaxies within the group are all able to interact.

reply to post by jiggerj


I'm not sure, but that article says they are dwarf galaxies, so maybe their gravitational force isn't as strong as Andromeda, despite being closer.

reply to post by jiggerj


Those Magellanic Clouds are contained inside of the huge gas cloud that surrounds the Milky Way.
Humongus, gassy halo found surrounding Milky Way

Now, There is no good info available yet. On how that cloud might effect interactions between the Milky Way and the two smaller galaxies, but I would not be surprised at all to learn that it was playing some type of role in this.

Originally posted by jiggerj
Several times I've read and heard that all the galaxies are moving away from each other. The way this is occurring is that the space between the galaxies is filling in with dark matter, thereby pushing all the galaxies further and further away from each other.

Question: What's wrong with the dark matter that's supposed to be filling in the space between the Milky Way and the Andromeda galaxy? Why are we on a collision course instead of being pushed away?

As you discovered, in fact the galaxies are NOT all moving away from each other. What is actually happening is that galactic clusters are moving away from each other. The Milky Way, Andromeda, and over 30 other galaxies are part of the same galactic cluster. The gravitational forces within a galactic cluster are stronger than dark energy forces within that cluster which is why Andromeda is moving toward rather than away from us.

Defining the Local Group of Galaxies

Galaxy Clusters

The Universe is expanding and galaxies are racing away from each other, as indicated by their redshifts. The further they are, the faster they are receding. But, on a more regional scale we find that galaxies tend to cluster together, drawn toward each other through the relentless pull of gravity. The spectra of these galaxies measured from within the cluster will show a blueshift, indicating that they are moving toward each other.

Also, I think you meant dark energy, not dark matter, which is why I referred to dark energy.

Originally posted by Arbitrageur

But, on a more regional scale we find that galaxies tend to cluster together, drawn toward each other through the relentless pull of gravity. The spectra of these galaxies measured from within the cluster will show a blueshift, indicating that they are moving toward each other.

Also, I think you meant dark energy, not dark matter, which is why I referred to dark energy.

edit on 1-10-2012 by Arbitrageur because: clarification

Thank you! I didn't know that.
And, I'll never get Dark Energy and Dark Matter right.

Originally posted by jiggerj
Thank you! I didn't know that.
And, I'll never get Dark Energy and Dark Matter right.

You're welcome.
I know the difference between dark matter and dark energy, but I too can say one when I mean the other, on occasion. I think it's called a "brain fart" though that may not be the correct technical term.

reply to post by jiggerj


Atomic activity generates waves in the dark matter once thought of as electrical carriers in
an insulating liquid. Thus the waves. Vibrations in a liquid can attract or repel.