Hi!
1. Galaxies are constantly evolving with time, and one manner that they do this is that they change their rotation. This is apparent from the knowledge we have of initially formed galaxies which are spiral like in the beginning, but over time transform into what seems like smooth, barren disks. Another difference is that some galaxies are younger than others, and the age difference is observed with younger galaxies having a higher number of stars that are also comparatively brighter, which makes these galaxies look blue. Older galaxies contain older stars, and older stars are seen to give off red light, which is why these galaxies seem red. Galaxies also undergo various physical and chemical changes, and older galaxies are more structured.
2. Galaxies are brought into formation with the major factor being gravity. The force of gravity is responsible for attracting various stars, collapsing gas, dust clouds and other dark matter into a structured body. Over time the organization becomes more defined. So basically, a galaxy starts off as clouds of dust and stars in space, which come into proximity with other similar clouds, and their subsequent interactions shape the structure of the galaxy.
3. Edwin Hubble, also known as the 'pioneer of distant stars', has contributed to out understanding of galaxies in that he was the first person to prove the existence of galaxies other than the Milky Way, through his studies on spiral nebulae (which were formerly thought to be clouds of dust and gas, but were actually galaxies). He also established the relationship of red shift of a galaxy (recession velocity) and the distance of the galaxy from the Earth - a directly proportional relationship, which is known as the Hubble's Law.
4. The Milky Way is found to be considerably larger, about ten times, than the average dwarf galaxy being 100,000 light years across in diameter. The Milky Way is home to 200 billion stars, and with the capacity (dust and gas) to give rise to billions more. The Milky Way is the biggest galaxy in the Virgo Supercluster, and is relatively old in comparison with most other galaxies within the universe.
5. Dark matter and gravity play a role in determining the structure of the galaxy and influences the environment over time. Dark matter contributes to the mass of the galaxy, and this affects the spin of the galaxy. The more the mass of a galaxy (or more massive a galaxy) the greater would be its rotation. Gravity is the acting force that holds the components of galaxy in place, such as the dust clouds, colonies of star, gas and dark matter. Gravity influences the structure in that it exerts and inward pull on the stars, resulting in the spiral arm structure of a galaxy, which over time tends to be more eclipse like.
Hope this helps!
