"Asimov" wrote in message
...
snip

If we were talking about inventing something then anything (or almost)
is possible. What I meant is that normal matter exhibits an attractive
gravitational effect but so does anti-matter. However, my question is
about a repulsive gravitational effect. For example is the newly
discovered acceleration of the BB expansion caused by an unknown
repulsive form of gravity or something else?

That is still open to question AIUI. WMAP went
some way to eliminating some theories

http://map.gsfc.nasa.gov/m_mm/mr_limits.html

If you track down the relevant papers, you may
will get more detail.

George