Then by all means, show us an observed black hole.
...and just in case you still have a problem with this information:
Here's the equation: Mp^2 = a^3
Translation: mass times period squared equals distance cubed (actually orbital axis cubed, which is where the "a" comes from).
The upshot: If you want to know how massive the object is at the center of our galaxy, all you need to know are the orbital period of a star revolving around it, as well as the star's distance from the object. Then you can solve for the mass.
Luckily, our infrared telescopes allowed astronomers to observe the orbits of those stars around the mystery object over a long period of time, which is what you see in both those videos. So we were able to figure out A) the stars' orbital periods and B) how far away from the object their orbits are. Which is how we discovered that the object is likely 4.1 million solar masses, and 6.2 light hours in diameter (roughly Uranus' orbit around the Sun). That means this object has crammed the masses of 4.1 million Suns into a space that's as small as Uranus' orbit. Holy freakin' moly. <<<<Only a black hole is capable of doing that.>>>>