I think you will find it is a blind assumption. You've not been to cite an experiment where I can get some sand/dust together and see it come together to form a bigger mass which is then capable of attracting more dust/sand.
I think you will find I gave you three examples of experiments you could do, one being to take your sand and rock outside earth's gravity, the second being to throw your rocks and sand in the air and watch them fall to the ground demonstrating that the gravity being exerted on them by the planet is larger than the gravity they exert on each other, and the third being to do a similar experiment with a magnet and some metal shavings because magnetism is stronger than gravity.
You are now saying I need an environment of zero-gravity to do this, I assume. Which then means you can't do this experiment on earth. Which then confirms it is blind assumption.
Do you doubt the existence of human space programs?
The stars that astronomers say are forming in clouds of dust - are they forming in an environment of zero-gravity? Think carefully about this.
It's not zero gravity, because the dust has its own gravity; but it's not a very large amount of gravity because the mass of the dust is very small. So that's why it takes such a very long time for the stars to form out of that cloud. If the cloud of dust was in the gravitational pull of a large object like an existing star or planet, it wouldn't be able to form into a star or planet, it would fall into the orbit or heart of the star or planet.