I found this easiest to grind out in the Casin system. The asteroids are white clouds. Some have rainbows too. The reason being, is that you can fly into or through them without suffering any damage. All you need to look out for are those that look like dark grey storm clouds as these can damage you.
The most effective weapons that I found to be of use, are Plasma Burner, Rocket Barrage or Laser Unicorn Attack Squad due to making quick work of them. Upgrading your weapons is also suggested which will make quick work of them pesky asteroid clouds. But upgrades will soon become incredibly expensive. So your best bet is to find a weapon you'd prefer to use, then upgrade that and crack on.