https://fivethirtyeight.com/features/juiced-baseballs/We CT-scanned and chemically analyzed baseballs from the last few years. Starting in 2015, the inside of the baseball looks different.
I'm not going to pull choice quotes from the article, it's worth reading the entire thing. It makes a very compelling case that up to half of the increase in home runs we've seen since the all star break in 2015 can be attributed to various small changes to the ball (the core, lower seams, etc).