I was going to comment on this as Marc, but you deserve a special Grammarnazi scoring for the irony of one of your comments.
how could they find out the age of the universe when they are not sure if the speed of light was constant during the big bang,
1) Capital "H" in "how." -1
2) I'm pretty sure that "big bang" gets capitalized. I'm not positive, however, so I'm not going to dock you any points here.
3) Question mark after "bang." -1
how are they sure that at the time of the bang the speed was not 3000 or a million times the speed of light.
1) Capital "H" again. -1
2) Question mark, not period, after "light." -1
Some scientists are retarded they think they know things, but they don’t.
1) Semicolon after retarded. -1
2) Okay. Let's stop right here for a second. You're currently at -5 on my grammar scale (-7 if I'm right about "Big Bang"). Are you really certain you want to call someone with (most likely) a doctorate a "retard" at this point?
God created the universe, there was no big bang and God is going to come back to judge it, so get ready and read the Bible and put your trust in Jesus Christ
1) I'd put a period after "it." -1
2) Get rid of "so" and capitalize "get." -2
Overall, you receive a -8 on my grammar scale. Stop calling people "retards" when you can't even use basic English grammar.