Technology

They're metal things sitting up there where the sunlight hits them. What you're seeing is the sun reflecting off them. Its like how you can look up and see a plane all bright and in the sun even at dusk when its starting to get dark on the ground.

They don't have lights, but they reflect light from the sun at certain times of day. Another way to think about it: these satellite can be experiencing broad daylight hours after the sun has set at the surface. Similarly, when you're seeing the Moon at night what you're actually seeing is daytime on the Moon and it's often enough to light up the landscape around you because you're looking at an object that is experiencing daytime while you aren't.

The question then becomes: can't the satellite be made darker? And the answer is they are already pretty dark. The moon, for example, has an Albedo (measure of reflectiveness) of 0.15, which is similar to asphalt and it can still dominate a night sky. SpaceX satellites have an Albedo of about 0.11, so astronomers are essentially having to deal with thousands of tiny and unpredictable Moons drifting across the sky. I can't find the article now, but I recall reading that the albedo would need to get down to 0.002 to become negligible; I just don't see that happening.

Thermal management is a huge issue for spacecraft. In atmosphere, the bulk of cooling for things like electronics would be convective, from transferring the heat into a fluid (air/water/etc) which then moves away with the heat. In space, you don't have a fluid for convective cooling, so your cooling is all radiative - essentially just emitting infrared energy. This is far, far less efficient - you need much more material and surface area to get the same cooling.

Dark objects are better at radiative cooling... unfortunately, they're also far better at absorbing radiative energy. Like the oodles of it coming out of the sun. That's why dark objects are dark - they're absorbing the energy. However, it also means that your thermal management is far more difficult because you're absorbing a lot more heat. It can be worked around, but it makes the spacecraft larger and heavier, which is the antithesis of space work. So spacecraft have traditionally tried to reject as much absorbed energy as possible, which by definition makes them reflective.