Where are they?

I know right. Why don't they just whip out their cellphones and call up nasa and be like "O SHIT WADDUP?!?"

this

i mean even then, you'd have to make sure that the "please respond memes" are back by friendly intentions and not a trap.

What if there is a civilization directly behind the core of our galaxy?

Really fuckin' far away, in all probability some of them rose to our level or greater and have since then gone extinct due to various causes.

Wouldn't we see them because of gravitational lensing?

Gravity is a social construct.

Sheer distance prevents it. If our galaxy was the size of a football field, earth's radio bubble would only be about 6" in diameter. That's how undetectable (and insignificant) we are.

Whats the signal to noise ratio on that bubble?

Doesn't work over such short distances for such low masses. You need galactic masses over intergalactic distances.