Question

what can be observed only by observing its effects on gravity? dark matter a solar nebula gas and dust

Explanation:

Dark matter does not interact with electromagnetic radiation, so it cannot be directly observed with telescopes that detect light, radio waves, etc. Its existence is inferred solely from its gravitational effects on visible matter, such as the rotation curves of galaxies and gravitational lensing. In contrast, solar nebulae, gas, and dust can be directly observed via their electromagnetic emissions or reflections.

Answer:

dark matter