Dispersion forces become significant only when molecules are:

1. Dispersion arises from momentary, correlated fluctuations in two electron clouds. 2. That correlation depends steeply on distance. 3. The forces are relatively weak and become significant only when the molecules are very close. 4. This is why dispersion matters in liquids and solids but hardly at all in a dilute gas. 5. Charge is not required, which is exactly why dispersion acts even between noble gas atoms. _Source: OpenStax Chemistry 2e (CC BY 4.0), Ch 10 'Liquids and Solids', sections 10.1-10.6_