The two forces that balance each other to keep nebula dust and gas from collapsing or flying apart are:

1. Gravity: This force pulls all matter towards each other, acting as a cohesive force that tries to collapse the nebula.

2. Internal Pressure: This force arises from the thermal energy of the gas and dust within the nebula. As the nebula collapses, the particles get closer together, increasing the pressure and temperature. This increased pressure pushes outward, counteracting the inward pull of gravity.

These two forces are in a delicate balance. If gravity is stronger, the nebula will collapse under its own weight, potentially leading to star formation. If internal pressure is stronger, the nebula will expand and disperse.

It's important to note: While these two forces are the primary players, other factors like magnetic fields and radiation pressure can also influence the stability of a nebula.