True, a pure GIMP alone can't do it. But they could account for most of the observed effects of Dark Matter, meaning we'd be very far from ever detecting any. A

True, a pure GIMP alone can't do it. But they could account for most of the observed effects of Dark Matter, meaning we'd be very far from ever detecting any. And an extra force (if one exists) might interact with Dark Matter but not to any detectable degree (or at all) with normal matter, leaving things in about the same position (you'd never produce Dark Matter from colliding normal matter).