The galaxy referred to in this context is the Milky Way, which is the galaxy in which our solar system resides. The primary reason we cannot see the entirety of the Milky Way from our vantage point on Earth is due to a combination of factors:
Size and structure: The Milky Way is a vast, disk-shaped galaxy that spans about 100,000 light-years in diameter. We are located within this galaxy, about two-thirds of the way out from the center. The vastness of the galaxy makes it challenging to observe its overall structure from within.
Interstellar dust and gas: The Milky Way contains vast amounts of interstellar dust and gas. These dust clouds and gas can obstruct our view of the distant regions of the galaxy. They absorb and scatter the light, making it difficult for us to see beyond these obstructions.
Intrinsic brightness limitations: The stars and other celestial objects within the Milky Way emit light, but their brightness decreases with distance. As we try to observe objects farther away in the galaxy, their light becomes increasingly faint and difficult to detect.
Light pollution: Light pollution caused by artificial lighting in populated areas can significantly hinder our ability to observe faint celestial objects such as the Milky Way. The brightness of the night sky in urban and suburban regions can overpower the dim light from distant stars and make it challenging to see the galaxy.
To get a better understanding of the structure and composition of the Milky Way, astronomers rely on various techniques such as studying the distribution and motions of stars, mapping interstellar gas, and using infrared, radio, and other wavelengths of light that are less affected by dust. Additionally, space-based observatories like the Hubble Space Telescope have provided invaluable insights into the galaxy by capturing images of distant regions unobscured by Earth's atmosphere and light pollution.