  Recent observations of near-infrared and X-ray flares from Sagittarius A*,
which is believed to be a supermassive black hole at the Galactic center, show
that the source exhibits about 20-minute periodic variability. Here we provide
arguments based on a quantitative analysis that supermassive objects at
galactic centers may be bubbles of dark matter axions rather than black holes.
An oscillating axion bubble can explain periodic variability of Sagittarius A*
and yields the axion mass about 0.6 meV which fits in the open axion mass
window. The bubble scenario with no other free parameters explains lack of
supermassive "black holes" with mass M<10^6 M_{Sun}. Low-mass bubbles decay
fast and as a result are very rare. We also found that the mass of an axion
bubble can not exceed 1.5\times 10^9 M_{Sun}, in agreement with the upper limit
on the supermassive "black hole" mass obtained from observations. Our finding,
if confirmed, suggests that Einstein general relativity is invalid for strong
gravity and the gravitational field for the bubble effectively becomes
repulsive at large potential. Imaging a shadow of the "black hole" at the
Galactic center with VLBI in the next decade can distinguish between the black
hole and the oscillating axion bubble scenarios. In the case of axion bubble, a
steady shadow will not be observed. Instead, the shadow will appear and
disappear periodically with a period of about 20 min.
