Are there any scientific proofs of existing ufo? has anybody seen it?

The correct answer is No.

As of now, there are no scientific proofs that there are UFOs, only conspiracy theories exist.

Sure. People see Unidentified Flying Objects all the time. They don't know what the object is, and that's why it's called "Unidentified". Then it goes away, and that's the end of the matter. There's no more to say about it. Sometimes you'll hear somebody say "I didn't know what it was, so it must have been ". That's what's called a perfect absurdity.

Gas will most likely  fill the container