1. The Milky Way is not a plane. At best, it can be compared to a flat plate. Space is three-dimensional and cannot exist in a two-dimensional plane. Its upper and lower parts are not necessarily vacuum. The vacuum is on the earth, taking the ground inertial system as the reference object. In thermodynamics, vacuum means that the pressure is lower than the standard atmospheric pressure. For example, there is a high vacuum in the condenser of a power plant. A vacuum is not when people realize that there is no zero air pressure. Extragalactic galaxies exist in the upper and lower parts of the Milky Way. The distance that humans observe with space telescopes is limited. For the universe, we are not standing on the ground, but like a planet suspended in the sea. Just like clouds in the sky, there are substances up and down, left and right. Now we can simply say that the upper, lower, left and right sides of the Milky Way are extragalactic galaxies, similar to the state of being wrapped. However, all this is moving and expanding.
3。 The upper north and lower south of the earth are subjectively defined by human beings, but the magnetic poles of the earth, that is, the north and south poles, exist objectively. We just artificially define north and south, just like a scale. We stipulate that 1 m is equal to 100 cm, and we stipulate that the critical state of water vapor is artificially added to an objective thing. As long as the magnetic pole of the earth remains the same, different angles will not appear in the southern hemisphere and the northern hemisphere.
2。 Taking the orbit of the earth as a plane, it is impossible to determine whether there are many stars in the north or the south, because stars have orbits, and the same star may be in the north for a while and in the south for a while. Besides, there are many stars that we haven't discovered, so we can't be sure how many stars in extragalactic galaxies can be discovered by releasing n rays from the earth, so the number of stars in the north and south is not absolute.