Question

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

Explanation:

Dark matter does not interact with light or other forms of electromagnetic radiation, making it invisible. However, its presence can be inferred through its gravitational effects on visible matter, such as stars and galaxies. Planetesimals, gas - dust, and solar nebulae can be detected through various electromagnetic means.

Answer:

dark matter