alistair wrote:

[...]
shouldn't we
look at the possibility that some forms of energy do not curve
space-time?

Of course we should. This is a *major* experimental program,
with easily a dozen groups investigating the question of whether
composition affects gravitational fields.

So far, though, not one speck of evidence has been found to
suggest that ``some forms of energy do not curve space-time.''
There are strong limits for electrostatic and magnetstatic
energy, strong interaction energy, and gravitational binding
energy, with somewhat weaker limits on kinetic energy and the
energy of the parity-conserving part of the weak interactions.
For these, we have experimentally ruled out the possibility
that they don't gravitate.

Did you have some other kind of energy in mind?

Steve Carlip