What is the universe expanding into

Nothing. There is no edge moving outward. Space is simply expanding.

Imagine the 2D surface of a balloon. As the surface of the balloon expands yet there is no edge moving outward. If you having a problem conceptionalizing this, keep in mind that if you walked in a straight line around the balloon, or any sphere, you would eventually end up back where you started. The same may be true of our universe, in which case our universe would be said to be "finite and unbounded". Or perhaps the universe stretches out to infinity and there is no curvature (curvature of our 3D universe bent into a 4th spacial dimension like how the balloon is a 2D surface bent into a 3rd spacial dimension).

*edit*
@Ashkan

The 2D balloon surface is expanding into an unseen 3rd dimension just as our universe may be a 3D volume expanding into a 4th dimension. It is unseen for the same reason peoples of antiquity believed the Earth was flat. There is so little curvature to the Earth's surface and we are so small in comparison that we assume the surface of the Earth stretches off into infinity in a straight line in every direction. The same goes for the universe. We are so small compared to the size of the universe that (if the universe is finite and unbounded) we don't notice the curvature.

Either way, the universe isn't expanding into anything. It is simply expanding.

actually i always ask the same question but no one is able to give any correct and convincing answer. all the people give the example of balloon. but balloon is in a space with is the earth's atmosphere. and the earth's atmosphere with the planet are in the space and that space is in the universe so this universe must be into something else and that thing into something else. you cannot expand something into nothing. they say that the space is created after the big bang but WHERE was this big bang? how can a big bang happen when there's no space? and where was this big bang? big bang started the time, but how could big bang exist in the first place if there was no time before it? how could ANYTHING exist if there was nothing before it? or the question is: how can you make everything out of nothing? if there was something, then the big bang wasnt the beginning.