ESPN's Sport Science commissioned a study into the composition of Major League baseballs. Thestudy compared game-used balls from before and after the 2015 All-Star Break based on x-ray imagery and the chemical composition of their cores.
Via Five Thirty Eight:
On top of the fact that the balls became bouncier as the core itself changed, previous research at FiveThirtyEight showed that they also became less air resistant. The decrease in drag is probably a result of a smaller, slicker baseball with lower seams. The change in air resistance could add an additional 5 feet to the travel distance of a fly ball. Combine all these factors together — a lighter, more compact baseball with tighter seams and more bounce — and the ball could fly as much as 8.6 feet farther. According to Nathan’s calculations, this would lead to a more than 25 percent increase in the number of home runs. Asked whether these changes in combination could have significantly affected the home run rate, MLB declined to comment.
In actuality, home runs spiked by about 46 percent between 2014 and 2017, which means that the changes to the ball could account for more than half of the increase.
It’s not just that the inside of the ball looks different — the chemical composition of the cores appears to have changed as well.
The 2017 season set an MLB record for collective home runs with 12 days left in the season. The record was obliterated, as the 2017 season saw 6,104 home runs. The previous high was 5,693 (2000). I think this is all the evidence we need. The Balls are definitely juiced. It's better for baseball to have a crazy amount of dingers.