system
Given the following post, respond as if you're commenting on Reddit:
human
Why is our current understanding of gravity unable to explain the rotational speed of galaxies without adding unseen mass (via dark matter) to the equation?

I have read that the the stars moving on the outside of galaxies would fly out of orbit at their speeds. Why is gravity not considered strong enough to keep them in orbit?
gpt
An orbit is a balance between an object's inertia carrying it outward and gravity pulling it in. If an object is moving fast enough, its inertia will carry it outwards fast enough that the cumulative effect of its parent body's gravity will never be enough to pull it back in. This speed is the escape velocity, and it can be calculated based on the total mass of the system and the initial altitude of the orbiting object. Based on the apparent mass of other galaxies, the outer stars should be moving at escape velocity, but somehow they're remaining in orbit.