A speculation I had recently: I've always been fascinated by black holes, and primordial black holes. The latter aren't known to exist but they're predicted by a lot of models and a bunch of JWST observations have increased the likelihood that they exist.
If primordial black holes exist, they might have been formed across the whole mass range. They're a candidate for at least some of dark matter for that reason.
But very small ones would already have evaporated, so the only ones left would be about asteroid mass or heavier.
That gets to my speculation. When a black hole evaporates (if we are right about how this works) it hits a runaway phase where it basically detonates, converting its remaining mass-energy into a shower of particles and radiation. The latter would include a lot of hard ultra-high-energy gamma rays. I've heard this put forward as a possible source for some of the very spicy photons we've detected.
If there was once a population of small ones that were evaporating, could this have resulted in a universe that until recently (in cosmological terms, so billions of years) was full of little evil bang-pop firecrackers that kept going off and spraying everything near them with ionizing radiation.
That could have more or less forbidden the evolution of complex life, technology, and spacefaring intelligences until "recently." (I'm aware of the other thing that might make us early in that regard, namely metallicity and early stars and planets probably being metal poor. Both could be true.)