It's from the Model G theory in Subquantum Kinetics, -- and it seems more believable than a finite amount of matter being made from a single blast of energy billions of years ago.
But physics is not about believing, it's about describing reality. And when we look at our universe, we are not seeing new matter being created. If the theory actually says that new matter is created while the the universe expands and the amount is not so small that it may have escaped detection, then the theory is wrong. And not because somebody believes or says something but because the theory does not match reality.
The universe is old, at least according to mainstream physics, even small changes would accumulate over billions of years and cubic lightyears. And we are good at measuring things, we famously measured the g-factor to a part in a trillion, detecting gravitational waves required detecting length changes on the order 1/1000th the diameter of a proton, 0.000000000000000001 meters. A quick search reveals that we know the energy density of the universe to better than half a percent. So what are the numbers? How much new matter is generated per unit time and volume according to the theory?
I'm still researching this, but it's happening at a quantum level, so I expect it to be slow. But like you said, it will have an aggregate affect over time.
Also, it sounds like you're interested. Why not read the book or watch the video lecture?
I am not interested, I am absolutely sure that the theory is total nonsense. I just wanted to make you realize that, preferably by making you discover it yourself by hinting at problems, not by simply telling you that it is wrong.