Answer to Question 1
Stars rotate around the galactic center in ways that are controlled by the way the mass of the galaxy is distributed. A star that has distributed matter inside and outside of its orbit will orbit according to one principle, a star that is orbiting outside of the mass will follow another (Keplerian) principal.
We observe that the orbital periods of stars follows the pattern of an orbit inside of a distributed mass, rather than changing to a Keplerian orbit pattern after crossing the boundary of visible matter. This means that there must be mass outside of the visible region of the galaxy. This mass does not emit or absorb photons of electromagnetic energy, so it is termed dark.
Answer to Question 2
Gravity is universal, so the Sun also produces tides on Earth. The Sun is 27 million times more massive than the Moon, but it lies almost 400 times farther from Earth. Tides on Earth caused by the Sun are less than half as high as those caused by the Moon. Twice a month, at new moon and at full moon, the Moon and Sun produce tidal bulges that add together and produce extreme tidal changes: At those moon phases, high tides are exceptionally high, and low tides are exceptionally low. Such tides are called spring tides. Here the word spring does not refer to the season of the year but to the rapid rising up of water. At first- and third-quarter moons, the Sun and Moon pull at right angles to each other, and the tides caused by the Sun partly cancel out the tides caused by the Moon. These less extreme tides are called neap tides, and they do not rise very high or fall very low.
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A land snail (Helix aspersa) with the opening that leads to its lung visible at the left.