In the universe, every celestial body and galaxy is subject to the universal gravitational pull, so a mass of matter will continue to shrink toward the center, and so will our Milky Way.

However, since the Milky Way is not static, it will rotate itself, and centrifugal force will be generated during the rotation. This centrifugal force will block the contraction, and a certain balance needs to be maintained between the two.

Since the direction of centrifugal force is on the rotating surface of the Milky Way, but there is no centrifugal force in the vertical direction of rotation, the contraction in this vertical direction will become very strong, and the direction of rotation will shrink to a certain extent due to the existence. The centrifugal force will stop, and finally, the vertical direction is narrower and the rotation direction is the wider shape, and eventually, the Milky Way becomes a flat disk.

Not just the Milky Way, but the Solar System and Saturn's rings are flat, mainly because of accretion, which is the collapse of matter by gravity from dust, gas, or (usually) dust-gas nebula the process of.

In a perfectly elastic collision, the colliding objects bounce around the same angle as when they came. Most collisions are inelastic, meaning they lose energy and the angle between their trajectories decreases after the collision.

In extremely inelastic collisions, particles stick together. This condition is common in tiny particles. Over time, these impacts release energy (in the form of heat and light). This loss of energy causes the nebula to physically shrink, as the loss of energy causes the dust and gas particles within it to slow down (and to orbit lower and lower).

But the impacts also "averaged" their trajectories. So, each grain of dust and gas inside a large, fluffy nebula would otherwise orbit in all directions, but during the accretion process, they would eventually settle evenly into the same plane of rotation.

Humans have always wanted to achieve interstellar travel, and this requires the ability to fly out of the Milky Way. Since the Milky Way is flat, why not fly up or down vertically on the galactic plane, isn't it easier to fly out of the Milky Way?

The first reason is the gravitational factor. In fact, no matter whether the spacecraft is flying up, down, or forward, it will be affected by the gravitational force of the black hole in the center of the Milky Way, and the gravitational force of flying in all directions is equal.

The gravitational force generated by the black hole in the center of the Milky Way is like A huge gravitational cover wrapping the Milky Way, and its gravitational force exists in all directions. So no matter which direction you fly in, a condition must be met, that is, to reach the fourth cosmic speed (120km/s, 430,000 kilometers per hour), only if this conditions to fly out of the Milky Way, otherwise you cannot fly out of the Milky Way in any direction.

The second reason: is the gravitational slingshot effect. What is the gravitational slingshot effect? The so-called gravitational slingshot effect is that the spacecraft makes full use of the gravitational force of celestial bodies to accelerate, save fuel, and adjust the flight direction.

Scientists believe that flying out parallel to the galactic plane is the best route because there are many stars and planets on this route. The spaceship can be filled with the gravitational slingshot effect of these stars or planets to achieve acceleration and fuel-saving purposes. If it flies up perpendicular to the galactic plane since there are few planets there, it is almost impossible to use the gravitational slingshot effect, so the spacecraft fundamentally Not enough fuel to fly out of the galaxy, you may run out of fuel halfway through the flight, and without fuel, everything is empty talk.