I can't get by the question .. expanding into WHAT?..
Is the Universe a balloon?..
There is something known as the Doppler shift. We experience it on earth with sound. We can tell if a train is approaching us or getting farther from us, because the sound waves "stack up" when it's approaching, and "stretch out" when it's leaving.
The same principle applies to light. The object that approach us rapidly will have a spectrum shifted towards the blue end (shorter or "stacked" wavelengths), while those receding from us will have their light spectrum shifted towards the red (longer, "stretched" out wavelengths).
When astronomers look at the Doppler shift, it is apparent that, overall, everything is flying away from each other, that is expanding.
The Big Bang theory speaks of an initial explosion that resulted in the formation of the universe as we know it. Thus, all of the universe looks like an explosion "cloud" -- hot and expanding.
In the mid 1960's researchers were able to measure the "background' temperature of the space. Instead of it being absolute zero (as one would expect in perfect vacuum) they discovered that it was about 2.75 degrees above absolute zero (the unit of measure is absolute temperature or Kelvin, not Fahrenheit, or Celsius). In other words, the space is still "warm."
As long as it is "warm," it will expand like a balloon. When it hits absolute zero, it should stop. But, the scientists are telling us that the universe is not only expanding but accelerating!
The problem is it has been expanding at a high rate of speed for some 30 billion years it is estimated and is, as our dear sister in Christ tells us, 156 billion light years wide (light travels one second from the earth to the moon, or about 160,000 miles/second). So this 'void' into which we are all hurtling is rather large. :)