Because galaxies were born spinning, and nothing in space stops them.
Logical explanation -----
*1. Everything in Universe Was Born From a Spin*
- Big Bang threw gas everywhere. Gas was not perfectly still. Small random movement.
- That gas cloud that made our galaxy - it was huge, light-years wide, and had a tiny, tiny spin. Like a slow merry-go-round.
*2. Ice Skater Effect - Conservation of Angular Momentum*
- That huge cloud collapsed under its own gravity to make stars.
- When big thing shrinks to small thing, it spins FASTER to keep its spin power same.
- Example: Ice skater - arms wide = slow spin. Arms close = fast spin.
- So huge slow cloud -> small galaxy = much faster spin.
*3. Nothing Stops It*
- On Earth, a spinning top stops because air and friction.
- In space, there is NO air, NO friction. So once it spins, it spins forever. For 13 billion years.
Why It Matters - The Mystery:
If we calculate speed, stars on edge of galaxy should spin SLOWER than center (like planets - Mercury fast, Neptune slow).
But *stars on edge spin FAST - same speed as middle !!!
That is why we discovered *DARK MATTER*.
- Visible galaxy mass is only 15%
- Invisible Dark Matter halo is 85% - Like invisible glue, adds extra gravity, holds fast-spinning outer stars from flying away.
> *Galaxy rotates because mother cloud was rotating, collapsed and sped up, and no friction to stop it.*
Without rotation, all stars would fall straight into center black hole. Rotation is what keeps galaxy spread out as disk.
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