They're called gnats or midges. They don't bite, they're more or less harmless to plants, they're entirely normal, there's barely any reason to swat them and even less reason to treat for them. The only way they can affect you at all is if you walk through a swarm and they get in your mouth and nose. Aphids munch on living plants directly, these guys usually don't.

They feed on dead plant matter on the very top of the soil, where grass will shelter them from the sun and wind, where we don't see em. They come up in a swarm only for mating purposes. Usually around sunset, but not always. Sunset is also when its easiest to spot them because of the angle of the light.

I have no idea why you haven't seen em before.