Sometimes my 5.5 month old will randomly cry, sometimes scream, while he's sleeping. He doesn't wake up. Just cries/screams for a second, then goes back to peacefulness. Is he having bad dreams or what could this be?
that is a completely normal behavior. Babys working on their daily experience through night nearly the same way we do. Sometimes if something really sets them up, they scream and cry as they do at daytime.
Babys and kids are not capable of controlling their emotions as good as an grown-up. So some events can trigger an emotional outbreak the kid can't control and it starts laughing, crying or screaming extremely hard.
That can happen at night too, when they are dreaming.
In a few weeks or month this behavior will slightly decrease but it not necessary needs to stop.
best regards Martin