Also, how much does gas expand?
The volumetric coefficient of thermal expansion for gasoline is about 0.000528 per degree F. It's dimensionless -- like cubic inches per cubic inch or gallons per gallon. If you multiply that by the 15 degrees (75F – 60F), the expansion is 0.0079 gallons per gallon.
Likewise, do gases expand? Heat causes the molecules to move faster, (heat energy is converted to kinetic energy ) which means that the volume of a gas increases more than the volume of a solid or liquid. However, gases that are contained in a fixed volume cannot expand - and so increases in temperature result in increases in pressure.
In this way, does gas expand infinitely?
Now you can answer for yourself what will happen when the gas molecules are put in infinite space. They will keep spreading further and further as they face no external pressure. That's all that will happen. With more and more time they will have spread to greater and greater volumes.
Why do gases expand to fill any available volume?
TL;DR - Gases expand to fill space because the particles of a gas have so much energy that they fly off in all directions, overwhelming the forces that hold them together.