It is actually indefinite. This is similar to how Swedish use words like the Internet without a definite form. (There is only one and there is no need to specify it further.)
Universe is a little bit trickier since there are theories and fiction with multiple universes where you may need to refer to the universe of a specific world.
Universum är lite klurigare eftersom det finns teorier och fiktiva historier med multipla universa där man kan behöva referera till universumet i en specifik värld.
I was looking for this point, too. Usually "s" becomes kind of "sh" when it follows r. Like in the word "universum". And usually this rule applies even throughout separated words. But between "är stort" it doesn't "work". Does the "t" affect the "s" more than the "r" or is it something else that happens here?