Note: I don't love my title and am open to any community edits that improve it!
"Really big deal" is of course relative, and it doesn't have to be a big deal for everyone but can be big for a specific domain, field, or group of people. It also dosn't have to be recent. Many "big deals" of the past have yielded things that are largely invisible to us now and that we take for granted.
What is something that is hugely significant but also seems to have no common presence in current discussions or understandings of the world?