Formation of Milkyway Galaxy.

 The Milky Way Galaxy, our spiral residence, formed around 13.6 billion years ago-about 200 million years after the Big Bang. Formation is a complex account pieced together through observations, simulations, and theoretical models while we are not able to see the very process itself. Now, here is a brief account of its formation as is understood today:

1.Early Formation: In the early Universe, minuscule density fluctuations in the cosmic microwave background radiation, formed during the Big Bang, started growing under the influence of gravity. These fluctuations, somehow mostly constituted of dark matter, formed dense clumps called dark matter halos. The Milky Way's formation started inside one such halo, which acted as a gravitational well and drew gas and dust.

2.<Gas Collapse and Star Formation>: By about 13.4 billion years ago, hydrogen and helium gas (with traces of lithium) started to collapse into this halo. Upon compression, the gas would have heated up to form the first stars: massive, short-lived Population III stars. These stars would have exploded as supernovae and enriched the surrounding gas with heavier elements such as carbon, oxygen, and iron, which were vital for the formation of later-generation stars and planets.

 3.<Protogalaxy Formation>: In the span of a few hundred million years thereafter, the collapsiging gas and the earliest stars gave birth to a protogalaxy-a messy, irregular blob. This protogalaxy grew by accumulating gas from the cosmic web (filaments of gas and dark matter) and merging with smaller protogalaxies. There were plenty of these mergers in the early universe that contributed to building up the mass of the Milky Way. 

4.Disk and Spiral Structure: The protogalaxy had begun settling into a rotating disk by about 10-12 Gyr ago due to the conservation of angular momentum. The disk then helped organize the spiral arms along which gas and dust sedimented, causing star formation to continue. The bulge of the Milky Way, full of older stars, was probably formed from rapid star formation and merging in contrast to the disk.

Key Evidence:---

1.Ages of Stars: The oldest stars in the halo and globular clusters of the Milky Way are roughly dated to 13.2-13.4 billion years, which means they set a lower boundary for his age.

2.Simulations: The IllutrisTNG project showed how galaxies similar to the Milky Way complicate a plethora of mergers in an apparent dark-matter-dominated universe. 

3.Gaia Data: The Gaia spacecraft's stellar motions mapping revealed some ancient merger events like Gaia-Enceladus, confirming that the Milky Way grew through further cannibalism.