It's not radioactive, it's been around for a while, it's dust with very high life.

Dust with high life?
What?

@vsolon7
probably a glitch or something.

@the-fall
life is a measure of the frames it takes for a particle to go away or something, im not quite sure.

give a link to the save?

EDIT: Fount it. No idea how the creator did that.

@Racer-Delux
https://powdertoy.co.uk/Browse/View.html?ID=175853 @Racer-Delux
he didnt he stole it from a guy i forgot the guys name ( i really hate my memory )

If you set its life really high, it turns invisible.
It glows if you change TMP