"universe" is expanding?

Try these books by Stephen hawking

As with explosion, celestial bodies continue to separate but according to Newton's law of gravitation, they also exert attraction on one another. It is thus rationaled that at one point the forces of expansion due to the initial big bang explosion will become equivalent to forces of attraction and then the universe will start wrapping up in the same way it expanded.

Check these out

**broken link removed**
https://news.bbc.co.uk/2/hi/science/nature/2346907.stm

It is true that the universe is expanding. How can we prove it? We can use the priniciple of Doppler effect to prove it. According to the Big Band Theory, the universe is expanding and all stars are moving from us. By examining the light reaching us from all stars, we do find that the wavelengths of hte spectral lines to be lengthened - Red Shifts (non-relativistic)
Expansion of the universe is being slowed by gravitational force. If it has enough energy, the universe will continue to expand forever. But if not, the expansion will eventually stop. Gravitational force will begin to draw the universe back again into highly compressed state. If this second alternative applies, the fireball will be recreated and the whole process will repeat.
I hope the above explanation will help you a bit. Feel free to contact me at any time.

The topic is too complex to discuss in short messages.