I read about it like 20 years ago and don't remember the details or never doubt something I can't understand. The way I remember, all element no matter how small is floating in a medium of space we don't know what it is and simply inertial / null medium / force... until identify.

Energy of burning star / sun pushes particles and when collide with another will generate gravity and attract more particles into planets over time, gravity is not permanent and will subside, Mars is thought to have gravity like Earth.