Answer:
(a) orbit the galactic center just as fast as stars closer to the center.
Explanation:
The orbital speed is defined as:
[tex]v = \sqrt{\frac{G M}{r}}[/tex]
Where v is the orbital speed, G is the gravitational constant, M is the mass of the supermassive black hole, and r is the orbital radius.
If the distance increase the orbital speed decreases (inversely proportional).
However, the results of the measurements of the orbital speed of the star in the outskirts of the Milky way show that in fact, they orbit the galactic center as fast as the star closer to the center.
Dark matter in the other hand was born to explain why stars that were farther from the supermassive black hole in the center of the Milky Way galaxy didn't decrease its orbital speed as it was expected since the contribution of more matter in that region is the only explanation to that weird behavior.
