[Lurker]

“Dark matter is estimated to outweigh normal matter by about 5 or 6 to 1.“

Then I should run over tons of it with my lawnmower. But, I don’t.

“But they don’t clump like normal matter,”

So it exerts gravitational force on “ordinary”matter but isn’t affected by its own gravitational force. Do I have that right?

“Observationally, we know something out there, with mass, must exist in order to have the gravitational pull that we see.”

Or your premise is wrong.

L

[Telepathic Intruder]

Then I should run over tons of it with my lawnmower

Yes, but in very small amounts. As I tried to indicate, it only exists in large amounts over a very vast volume of space.
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So it exerts gravitational force on “ordinary”matter but isn’t affected by its own gravitational force. Do I have that right?

No. It doesn't clump because it has no electromagnetic or strong nuclear force, which is what causes matter to clump due to friction. Things like the earth are held together by gravity, but only because friction slows atoms down as they clumps together, rather than flying past one another.
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Or your premise is wrong

That has always been a possibility to cosmologists. Either Dark Matter exists or we don't completely understand the full nature of gravity over great distances. But one thing is certain: visible matter in the universe doesn't account for the gravitational interactions that we are observing.