Da big bang

[Ironside]

Well, this is the science forum, so I don't understand what God is doing here

I for one, must admit that I don't know too much about this Big Bang theory.

What made scientists come to this conclusion?

Is the universe, according to scientists, still expanding after the big bang?

Which tools/methods do scientists use to measure this?

Sigurd is covering the red-shift pretty well and this explains most of your questions. In principle the lightwaves themself streches out with expanding space.
The biggest proof of Big-Bang instead of an evergrowing universe is the backround radiation. Basically when the universe was created it was very hot and then after cooling down to about 3000Kelvin, the universe stopped being a plasma (everything is ionized) and became transparent, creating the first light that could travel some distance. With time the wavelength has been stretched out into the form we see today on about 3K (the universe has then becamed about 1000 larger since then).

A plasma isn't tranparent because the photons will be immidiatly absorbed and then released again in a plasma, this is why the sun looks massive for example.

Will the universe one day, start getting smaller again? I mean, will it implode, only to explode (a new big bang) once again? Is it possible that there is a gigantic perpetuum mobile of imploding/exploding (Big Bang) of the universe?

As for the momment it seems that the universe is expanding faster and faster (this is observed with fairly high certaincy), driven by the dark energy (that's basically "if we put a number into this equation, then the equation will follow what we see"), that scientists have no idea what it is. Simply put as it is now it basically says that the more vacuum that's created the more energy that will push all objects away from eachother will exist.

As you can guess don't be surprised if that field ends up completly rewritten within a few decades.

According to scientists, does the universe stop? Is there a limit? Is that limit all planets, rocks, stars, whatever is floating around and beyond that, there is just space, with nothing in it?

Short answer: None got any idea.
Longer answer: The Universe is the boundry of our physical laws and also time as we percive it so it's pretty hard to say what's outside, or if outside actually can exist.

I'm intrigued by black holes, but I must admit that I don't know very much about them.

Short note, it's an area in space where the gravity is so strong that things needs to travel faster than light to escape the gravity (at this point matter as we know it is destroyed and what is then left is unknown). It can be of any size, but quantum mechanics tells us that the very small ones will vaporize very quckly by the Hawking radiation.