It happened gradually. Never when he was looking at it, never exactly the moment he turned away.
It grew. The green-ish mold-like whatever grew. So slowly, it was like watching the Tar Drip experiment. Again. It grew floating inside a near-absolute vacuum in a spherical glass container, with nothing to support its growth.
Well, there was sunlight, but no matter how efficient it was, it couldn't possibly synthesize matter from that.
The worst part was that when he released the vacuum, the particles scattered everywhere. All he could then was to reinstate a vacuum in the container and hope some of it stayed inside and would start growing again.
He needed it to grow big enough, to show shapes and patterns too complex and solid for a gas to form, and to prove something was really growing.
But it was growing. So. Slowly. He couldn't even measure the rate of growth with any accuracy.
Somehow, this mystery substance, whether it was a mold or a gas or whatever, held the secret to fusion power, or maybe neogentropy.
And all he could do was wait.