Can particles smaller than Planck Length be detected? It is my understanding, that anything smaller than a Planck Length cannot interact with photons, because photons with such small wavelength are massive enough to collapse into a black hole (source). Particle of such size would not interact with electromagnetic forces, but what about the three other fundamental forces? If this particle existed and was abundant in the universe but not uniformly distributed, could we be able to observe it's effect or could we build a device that could detect those particles?
I'm interested in this question, because dark matter famously doesn't interact with electromagnetic force and have a gravitational pull.
 A: Counter-question: what does "smaller than the Planck length" even mean, leave alone, can such particles even exist in the first place?
"smaller than the Planck length" is ill-defined precisely because of the meaning of the Planck length: at the latest at those scales, our concept of space and time (even if warped) will break down.
Even in the context of well-established physics, the "size of a particle" is an odd concept, due to the particle-wave-duality. Particles are not billiard balls, particles are quanta of their respective fields. Electrons for example are considered point-like, which is to say, smaller than any scale that we can probe, which in turn is still many, many orders of magnitude larger than the Planck scale.
Concerning your connection with dark matter, yes, dark matter could be made of quanta more massive than the Planck mass. Again, since the concept of the usual "particle" idea breaks down at the Planck scale, such heavy dark matter candidates are thought of as composite objects and thus typically have dimensions much larger than the Planck length.
A: I will make the comment into an answer:
Note that the standard model of elementary particle interactions is a quantum field theory where the particles in the table

are axiomatically assumed to be point particles (photon included) .
In the abstract of a book :

Quantum Field Theory of Point Particles and Strings (Frontiers in Physics)

.....

Part I of this book follows the development of quantum field theory for point particles,

A point particle has zero length by construction.
Considering that the model fits the preponderance of experimental data with these particles, and is very predictive, one can consider their "length" measured,and equal to zero ! The particles are surely detected in experiments.
