Doesn't need one, you insist that theories are beliefs, perhaps people believe the theory but unlike a belief in God, there is irrefutable evidence that supports theories. God requires a leap of faith, the theory relies on evidence.
You can't answer because it doesn't exist. I can directly measure the gravity of a bowling ball with little more than a piece of string. Why can't anyone directly measure Dark Energy? Because it doesn't exist.
Do you think Dark Energy is irrefutable evidence of the Big Bang?