paravir1996: Because there is wind! The dust rides this wind like a paper airplane, except it doesn’t need nearly as strong of a wind to stay up for a long time.
Imagine the game I am sure you have played with a balloon and keeping it from touching the floor. The dust is the balloon, and the wind is the people kicking/hitting the balloon to keep it up. Except this happens at really tiny sizes. Because we are huge to the dust, we don’t feel this “punching” from the wind, but the dust most certainly still does.
LieutenantCheeze: Generally dust has a density just slightly above air causing it to fall downwards. However air is made of various gaseous particles. The dust collides with these particles on its way down causing it to slow down, and appear as if it is floating