There is a Universal observer called God.
It is God's knowledge of Himself that cause
gravity. An Absolute Divine Observer, Creator
of Heaven and Earth, is the cause and the effect
of Gravity (Einstein is just a clown when compared
with God).

Proof:
Consider a fermion, say an electron, whose rest mass
is m_e. Now we ask, how much space can than electron
occupy? We pick up Pauli exclusion principle to see that
electron, even being a point-like particle, must occupy a
space. The space (volume) being occupied by that electron
can't be occupied by another particle simultaneously.
The Awareness of God tells us that each spatial point of the
flat 3-D manifold must be endowed with a real number in the
range [0,1]. Those real numbers, called occupation probabilities,
when integrated over a finite volume, yield the amount of volume
reserved to that electron with mass m_e. If the volume is the
differential dV around the center of mass m_e it would yield
p=1, but if dV is defined at infinite distance from that center of
mass it would yield p=0. So, if, for a given finite volume V, there
is a occupation probability p, then there must be a vacant
probability
p' = 1 - p. For that volume V, this means there is a free volume
V' = (1 - p) V, available for other particles, other than that
electron,
to travel through it. This vacant (free space, empty space) room is
what Einstein tried to guess as curved spacetime. Now, we can
see why a mass can 'curve' spacetime. But, God is Greater than
Einstein, He is the Cause and the Effect of Gravity. He knows not
only how spacetime is curved by masses, but the reason why it is.
What Einstein was even unable to see is that if a spacetime is
curved in some interval, then there must exist a complementary
curved interval reserved to the cause of that curvature, it is saying
reserved to the mass/energy. The vacant probability (p' = 1 - p) is
equivalent to the metric tensor in GR. But, God's knowledge is
Immense, and He knows Newtonian gravity is the correct theory of
gravitation revealed by Him to Newton.

God said: Let a space be reserved for fermions in Heaven and Earth
as

p = 1 - exp(V/c^2)

And God also said: Let a space be reserved for light in Heaven
and Earth, too, as

p = 1 - exp(2V/c^2),

where V is the newtonian gravitational potential, revealed by Me
to Newton. And God said, Let there be light: and there was light.
Once God said that, He remained at rest forever as the Absolute
Divine Observer of His Creation.

Let us pray to God, the King of all Creation! :-)