Dark Matter appears to have more in common with phenomena related to spatial geometry then a particle. I thought in General Relativity, space can be curved without the presence of matter so gravitational lensing does not imply there is matter present but that the space in a region is curved. If Dark matter has more characteristics related to spatial geometry, why is it referred to as a kind of exotic particle (WIMP).
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You're right that gravitational lensing, and really any large-scale stuff involving gravity, tells us directly about spacetime rather than the stuff out there.
But spacetime, according to GR, is constrained by the mass, energy, momentum, pressure, and shear. The equation we write down is $$ G_{\mu\nu} = 8\pi T_{\mu\nu}, $$ which unfortunately means almost nothing if you don't already know GR, but it just feels so good to write down people like me can't help it sometimes. Basically, the left side encompasses curvature and all that, while the right deals with the "stuff." Inferences about curvature therefore tell us something about the "stuff" there.
Our observations tell us there is a lot of "stuff" that has mass, moves slowly, and does not interact with light. We call it dark matter. Once you believe there is something with mass out there, quantum mechanics tells us we can probably think of it as a particle (i.e. an excitation of a quantum field, to be pedantic) on some level.
Beyond this, our models of cosmology work incredibly well when we postulate that there are particles out there that have mass and only interact via the weak force. These models include everything from the details of the cosmic microwave background to the clumping of matter into galaxies on large scales. Could you come up with some other explanation? Perhaps. But there is a lot of evidence coming from different directions pointing toward WIMPs of some sort.