Campaign finances, where does the unspent money go?

So in the wake of all this you need to vote to save the country and we all didn't so now we're all gonna die or we all did and are going live prosperously for eternity, depending on your side.

I was looking at this site and noticed that each person who ran and raised money didn't really spend all of it. Sometimes to the tune of $1 Million+. I am assuming that all the money in the spent category includes payment of the individuals working on the campaign. Where does the left over money go, who keeps it? Win or lose there always seems to be unspent campaign finances.

No, I didn't google this question as I find it more fun to ask reddit, mainly for the jokes, but I'll also get a straight answer I'm sure.