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 emit, absorb, or reflect light, and its presence is inferred from its gravitational effects on visible matter, light, and the large - scale structure of the universe. Solar nebulae, gas and dust, and planetesimals can be observed through other means like direct imaging or spectral analysis in addition to gravitational effects.

Answer:

A. dark matter