We can't have hydrogen blimps because there's a very strong chance that a car with a sparkler taped on it will dukes of hazard style ramp up into it and destroy everything.
Madness string 33194
In 2009, researchers found a cluster of galaxies moving at an extraordinary speed towards a small patch of sky between the constellations of Centaurus and Vela. Some unknown force is tearing away chunks of the galaxy, and astronomers still have no clue what exactly is causing it. It's been named 'Dark Flow.'