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, making it invisible. Its presence is inferred by observing its gravitational effects on visible matter, such as stars and galaxies. Solar nebulae, gas - dust, and planetesimals can be detected through other means like electromagnetic radiation.

Answer:

dark matter