Final answer:
The comparison of luminous matter in the Milky Way to the tip of an iceberg indicates that what we can see in the galaxy is only a small part of its mass, with the majority being dark matter that is invisible to our instruments.
Step-by-step explanation:
The text compares the luminous matter in the Milky Way (MW) to be much like 'the tip of an iceberg'. This analogy is used to convey that the visible matter that we can observe, such as stars and nebulae, is only a small, visible portion of a much larger mass of invisible dark matter, just as the visible tip of an iceberg is only a small part of the entire iceberg, most of which is submerged and not visible. The bulk of the MW's mass is made up of dark matter, which does not emit, absorb, or reflect light, and is, therefore, invisible to current astronomical instruments that detect electromagnetic radiation.
Considering that the luminous matter in the galaxy is attributed to around 1011 stars with an average mass about 1.5 times that of our Sun, and that dark matter is believed to be 10 times as massive, it becomes clear that what we see is only a tiny fraction of the galaxy's total mass. Studies indicate that dark matter and luminous matter are closely associated, with dark matter halos extending beyond the visible edges of galaxies. Furthermore, observations across the electromagnetic spectrum confirm that assessing astronomical phenomena based solely on visible light can lead to an incomplete understanding of the universe's complexity.
As we examine large clusters of galaxies, we find significant concentrations of dark matter, suggesting that these regions are gravitationally influenced by the dark matter's mass. Meanwhile, areas with an absence of galaxies, known as voids, are also absent of dark matter, implying a direct correlation between the presence of luminous matter and dark matter. The discovery that the Milky Way and other galaxies contain vast amounts of invisible matter underscores the importance of using instruments capable of detecting various wavelengths beyond the visible spectrum, to gain a more complete picture of cosmic structures and phenomena.