[Paul R.]

If it doesn’t exist, then what is causing the observed gravitational effects?

Note that observation of gravitational effects is a very well developed science, for a long time. It located Neptune and predicted the existence of Pluto, for example.

[thoughtomator]

> If it doesn’t exist, then what is causing the observed gravitational effects?

Step 1: discard the pseudoreligious assumption that all forces are gravity.