Here's a more technical answer than your mother =
There's 3 theories when it comes to the universe: Open universe, closed universe and flat universe. I know piss all about flat universe so let's forget that one.
The Closed universe runs on the assumption that as the universe expands, the rate of matter created will increase until there becomes enough mass to pull everything back into the centre causing another big bang. And the universe sort of repeats or whatever.
The most common theory is the open universe. This is a universe where everything expands forever and the amount of matter that will be created isn't enough to pull the universe back into the centre. As the time in the universe goes on, stars will form and die leaving behind black holes, neutron and red dwarves. After all the red dwarves use all their energy they become white dwarves (theoretically). Due to the dwarf and neutron stars using all their energy, they will turn into their constituent sub atomic particles and due to Hawking Radiation, the black holes will also decay. And then we get to the part where all the remaining matter: photons, neutrinos, electrons and positrons, are so far apart from each other they can't interact and the universe expands forever. What a fucking waste.
Scientist say all that will be left is this picture:
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