M 84 - Galactic nucleus and... a Black Hole?

Is this "almost artistic graph" the signature of a supermassive Black Hole in the center of distant galaxy M 84 (based on data recorded by Hubble's new Space Telescope Imaging Spectrograph (STIS)?. The presence of a Black Hole can also be revealed by watching matter fall into it.
In fact, material spiraling into a Black Hole would find its speed increasing at a drastic rate. These extreme velocity increases provide what we call a 'signature' of the Black Hole's presence. The STIS data show that radiation from approaching gas, shifted to blue wavelengths left of the centerline, is suddenly redshifted to the right of center indicating a rapidly rotating disk of material near the galactic nucleus. The resulting sharp S-shape is effectively the signature of a Black Hole estimated to contain at least 300 million solar masses. Now the question is: do all galaxies have central Black Holes? And, if "Yes", then "Why"?