It's possible that there are bots playing that haven't been discovered yet.
You're most likely to find those in HU-games as those are "easier" to be more or less break-even at.
Pokerstars themselves cheating? Not in a million years.
The Absolute Poker scandal a decade ago made sure that NO site would ever make that possible again. In fact, Pokerstars only sends other people's hand info to your computer at showdown. It's impossible to know what your opponent has in advance (other than having a really good read).
With the amount of rake they earn, why would they need to cheat? They earn money with every hand that gets played in cash games, and every entry in a tournament or SnG. They make even more money from Spin&Go games (yes, even with the million dollar ones that happen so rarely that they made many millions more first).