Should "could have formed" be "could have been formed"?
3. The same remarkable circumstance applies to the formation of heavier elements. If the strong nuclear force that holds together protons and neutrons had been even slightly weaker, then only hydrogen could have formed in the universe. If, on the other hand, the strong nuclear force had been slightly stronger, all the hydrogen would have been converted to helium, instead of the 25 percent that occurred early in the Big Bang, and thus the fusion furnaces of stars and their ability to generate heavier elements would never have been born.
Re: Should "could have formed" be "could have been formed"?
The active form is correct, but you could also use the passive.