To be "in denial" usually means that you are ignoring something that is obviously true. If you are "in denial" about something, then you choose to believe it's not true, even though there are good reasons why you should believe it is true. The sentence means: "If you do not believe that there is life out there (on other planets in the universe), then I think you are ignoring obvious reasons for believing that there IS life." The speaker believes that there is life on planets besides Earth, and he/she thinks that other people are being foolish if they don't believe so too.