Inflation
From what I gather inflation was the cause of the big bang and the expansion was the result although signs of inflation are in the gravational waves in the explansion. It's above my grasp at times so I could be wrong.
From the Space.com article:
Still, there is much more to learn about our universe's first few moments. For example, astronomers still have no idea what the substance that propelled inflation — dubbed the "inflaton" — actually is, Loeb said.
"It's not yet a victory of theoretical physics that we see evidence for a process that took place early on," he said. "We really need to understand what this substance — this inflaton — is. And until we do that, it's just like dark matter or dark energy — we give it a name, but we don't know what it is."