Final answer:
In our galaxy, clouds of luminous gas and dust are called nebulae, some of which can be seen as reflection nebulae. These regions, along with galaxy clusters that emit X-rays, are part of the large-scale luminous matter, but they also contain an even larger amount of invisible dark matter affecting the movement of stars and light. Correct option is d.
Step-by-step explanation:
Clouds of luminous gas and dust in our galaxy are called nebulae. These are regions in space where interstellar gas and dust have condensed. Some of these nebulae are reflection nebulae, which scatter light from nearby stars and become visible to us. When discussing large structures such as galaxies and clusters of galaxies, it's important to note that there is a huge amount of both luminous and dark matter.
While we can see the luminous matter as stars and nebulae, dark matter is detectable through its gravitational effects, such as the faster movement of stars in galaxies and the gravitational lensing of light around galaxy clusters. Galaxy clusters, including those with thousands of galaxies like Virgo and Coma, can emit X-rays detectable by observatories due to the extremely hot gas present between galaxies, which is heated by the cluster's gravity.