Mood Swings and Diet. Fish and salad dinner. You probably feel a little guilty when you eat a hamburger, knowing that the meat is bad for your heart. It turns out that diet affects your mental and emotional health, too. Diet and physical health are integrally tied.
The Omniverse is the collection of every single universe, multiverse, megaverse, dimension (alternate or pocket) and realm. This includes not only Marvel Comics, but also DC Comics, Image, Dark Horse, Archie, Harvey, and every universe ever mentioned or seen (and an infinite amount never mentioned or seen) including our own world.
My theory is that IT is the negative energy and the evil spirit within the macroverse/The deadlights and that pennywise/The entity the group and the audience sees is an actual eternal alien monster or demon that consumes children and is the portal between our universe and his evil spirited version of the macroverse.