First of all, the demonstrative pronoun is "there". "Their" is a possessive pronoun. And "no" is a negative particle -- to believe something that is a fact is "know".
Welcome to EvC.
-
But, atheists say that the universe has always been here.
Well, in a sense this is true: since there was no time "before" the universe existed, then there was no "before" before the universe. Hence, for any time you can specify, the universe existed. Yet, the universe probably has only existed for a finite amount of time. Is that a mind blower or what?
-
how did it just suddenly appear?
This may be unanswerable in principle. "How" implies processes but I don't see how there could be a "process" before the universe itself existed. But this whole topic is a pretty bizarre one -- personally, I don't think we even have the proper concepts in place to begin to discuss it.
-
I have asked atheists and they say it just did, they don't know why.
How is not knowing something a weakness? I don't know anything about how my ancestors arrived to North America. Yet I have a pretty detailed history of their travel from Ohio to Kansas to Oregon to Alaska during the 20th century. Just because you don't know some single detail doesn't mean that you don't know anything at all.
At any rate, I don't think it's possible to know why or how the universe came to exist -- I don't think that the questions even make any sense. Me, I think the universe simply exists -- it poses fewer conceptual difficulties than any of the alternatives.
If I had a million dollars, I'd buy you a monkey.
Haven't you always wanted a monkey?
-- The Barenaked Ladies