If you saw a line of dots of light crossing the night sky, silently and without blinking, it was almost certainly a Starlink train: a group of SpaceX satellites launched a few days ago that are still travelling together, one behind the other.
Starlink is SpaceX's satellite network for internet from space. It has thousands of satellites circling the Earth hundreds of kilometres up, and SpaceX keeps launching more several times a month.
Each Starlink launch carries a group of around two dozen satellites at once on a Falcon 9 rocket. When they are released they sit in an orbit lower than their final one and all follow the same path, like the carriages of a train. Then each satellite fires its own engine and slowly climbs to its final orbit. As they climb they spread out, and after a few days the line breaks up.
Satellites have no light of their own: they shine because they reflect sunlight. That is why you only see them shortly after sunset or shortly before sunrise, when your sky is already dark but they, much higher up, are still in the sunlight. In the middle of the night they enter the Earth's shadow and vanish all at once.
Once they reach their final orbit they are farther away and spread out, so you can hardly see them with the naked eye. SpaceX has also darkened its satellites so they bother astronomers less. That is why the best chance is right after a launch, while the train still flies in a line and lower down.
A Starlink train is several identical dots of light in a line, moving at the same pace, without blinking and without any sound, crossing the sky in a few minutes. A plane has coloured blinking lights and you can usually hear it. The International Space Station is a single, very bright dot. A shooting star is a flash that lasts a second.
Almost always a Starlink train: a group of SpaceX satellites launched a few days ago that are still travelling together. They look like a line of dots of light silently crossing the sky, shortly after sunset or before sunrise.
No. They are SpaceX's artificial satellites, and you can know in advance where and when they will pass. This site works it out every day from the orbit data SpaceX publishes.
A few days. After each launch the satellites climb to their final orbit and spread out, so the line keeps stretching until it no longer looks like a train.
A few minutes. The train appears on one side of the horizon, crosses the sky and suddenly fades when it enters the Earth's shadow or disappears on the other side.
No. They are hundreds of kilometres up. At the end of their life they come down and burn up in the atmosphere.
After each Starlink launch. On this site you can check, for your location, whether there is a train tonight and when it will pass.