So EVE Online is a massive MMO, all players are on the same set of servers in the same world. The game has been going nonstop for about 10+ years now (I devoted about 6 years of my life to the game, sadly)
Anyway, there is a massive battle taking place across several systems. It is a massive PVP Action with around 4500 players duking it out in a few different systems, although they are primarily focused in one system.
In real world money, around $20K has been lost so far.
Basic reasons for the fight (Swiped from Reddit):
They are specifically fighting over one space station/ hub, that is home to about 43,000 players. The station houses the ships, supplies, and assets for the largest alliance in the game.
There are a ton of Twitch streams up of people involved, and it is a ton of fun to watch it all play out.
Another REddit post that goes into huge details on this and really makes me miss playing the game: