Andromeda and mikyway will collide?

One problem you seem to be having is thinking that the Big Bang was like a typical explosion where things are flung through space. This isn't what is happening. In the Big Bang, it is space itself that is expanding. This is happening everywhere, so there is no center for the expansion. The way that the expansion happens is that galaxies farther away from each other are moving away from each other faster. For each megaparsec (which is about 3.26 million light years), the velocity due to expansion is 70 kilometers per second.

But, on top of this expansion, the gravitational interactions of the galaxies have to be taken into account. This causes a 'peculiar motion' that is added to the overall expansion. Well, gravity is attractive and is stronger for closer things, so galaxies closer together tend to feel more gravitational effects. In particular, the perculiar motions tend to be on the order of 200 kilometers per second or so.

Now, if you comnpare the overall expansion with the perculiar motions, you will find that galaxies that are closer than about 3 megaparsecs (around 10 million light years) can have perculiar motions that outweigh the expansion. This is what happens with the Andromeda galaxy (and a couple of others, by the way). Separating the two effects is not so easy: you have to know the distances to galaxies that are far enough away that the perculiar motion isn't such a big piece of the overall motion. THis was actually the biggest problem in figuring out the rate of overall expansion. Even galaxies 60 million light years away can have significant amounts of perculiar motion.

If you will look at some of the responses to a similar question I asked, you will get a better handle on what is meant by expansion of the Universe. Matter in the Universe is not traveling away from a center. Rather the matter is standing still subject to gravitational pull from other matter. It is the space itself that is expanding causing matter that is not in a gravitational relationship to recede. Andromeda is coming towards us because this galaxy is so close to us that it has a gravitational bond with our galaxy which is pulling it towards us faster than the space between is expanding.

The theory of the Big Bang was developed from Hubble's observation of an increasing red-shift on galaxies farther and farther away. Note the red-shift is not evident on galaxies within our own local group, just on distant clusters and superclusters.

Given this data, it was theorized that if distant galaxies are all moving away from us at ever increasing speeds the farther out/back we look, they must all therefore originated at the same point. Of course, Hubble himself disputed the cause of the red-shift, citing there could be another reason for the observation aside from Doppler shift.

Cosmic background radiation was not a foundation for the Big Bang, however, it was a predicted occurrence later supported by actual observations. The thing is, the radiation can also be used to support an infinite universe and other theories. The problem is that someone predicted it would be there to justify Big Bang and it was found.

If someone had theorized the amount of radiation to support steady state/infinite universe theory first, Big Bang would not be as widely accepted today as it is.

yes the universe is expanding. most of the galaxies are moving away from us. pretend you are able to observe the whole universe at once. you would then see it expanding. as you begin to look closer you would see various clumps of matter all moving away from each other. as you look closer at one of these clumps of matter you would see they are made up of hundreds of galaxies moving around each other as they move away from the other clumps of galaxies. the reason this is occuring is these say 300 or so galaxies making up this particular clump are close enough so that their respective gravitational forces are effecting each other. causing them to orbit each other. A couple of the galaxies will even collide because they are so close to each other due to gravitational attraction. 30 or so of these galaxies may be all within say 40 million light years of each other, then the nearest galaxy to after these 30 or so our counted is 200 million light years away, the whole clump might all be within 500 million light years, and the nearest galaxie to them is part of a dif clump 2 billion light years away. so as a whole, each of these clumps of galaxies are moving, expanding away from each other. but the clump itself is orbiting and in some cases moving towards each other because they are close enough together for their gravities to interact with each other. the timescale of this interaction is immense: It will be 3 billion years before the milkyway and andromeda galaxy begine to collide, the collision itself will last a couple of hundred million years. I hope I helped aliviate your confusion.