From a scientific perspective, we don't know why the universe exists. The Big Bang Theory provides an explanation of how our universe started and evolved into its current form, but it doesn't address why the Big Bang happened in the first place or why there is something rather than nothing. Many theoretical physicists are exploring various hypotheses, such as the Multiverse Theory (which suggests that our universe might just be one of many), but these remain largely speculative.
From a philosophical perspective, different answers have been proposed. Some philosophers argue that the question might be misguided or unanswerable because it assumes that there's a reason or purpose behind the universe's existence, which might not be the case. Others have pondered on existential ideas, suggesting that the purpose of the universe could be tied to the purpose of human life itself, though such perspectives vary greatly.
From a theological perspective, different religions have different answers. Many propose that the universe exists because a divine being or beings (God, gods, etc.) wished it so, and the universe serves some kind of divine purpose.
In conclusion, the question of why the universe exists is a deeply profound one without a clear, definitive answer. It's a question that continues to motivate a great deal of scientific, philosophical, and theological exploration.