I don’t have to “account for dark matter” and positing a big brain in the sky doesn’t do anything but move the question line back one step.
The assumption that “someone must have created this” comes from the idea that nothing can exist without having been created. This leads to the question “Then who or what created God?” If the answer is “no one,” then life can exist without having been created, which undercuts the idea that the universe must have been created if it is based only on the assumption that nothing can exist unless it was created. Positing a creator god only leads to a cul-de-sac where the answers become “because God wants it (apparently)” and that answer is not really an answer, just an evasion.
Actually, the argument is not that "life" cannot exist without having been created, but that you either give Divine attributes to matter, that of self-existence and intelligent design, or you allow for a actual Being or beings that do. The universe does not evidence the former, but the latter does since it evidences order and design.
Even extraterrestrial theories are more respectable than blind, purposeless chance, but once you allow for that you should allow for a creator God, but which is typically not tolerated even as a hypothesis by militant atheists.
The assumption that “this stuff has always been here” comes from the idea that a GOD does NOT exist.
This leads to the question “How long has it been here?"