In the double slit experiment, it is important whether a particle is observed or not.
Does observation mean I illuminate my particle actively, making photons interact with it and look what comes back, or can I see it because the particle emits photons?
To describe a macroscopic equivalent:
Is it like seeing a white ball fly by, because I have ambient light, and I would not see it if I switched the light off,
or like seeing a glowing ball fly by, and still see it if I switch the light off?
Usually, elementary particles are not detected using light, nor photons in general. They are detected by putting a "detector" on their path. The particle transfers some of its energy to the detector, in some form. In some cases, the particle is able to cross the detector without loosing too much energy and without changing too much its speed; in other cases, it can be completely stopped.
Maybe, the most simple example is the photographic film. Particles with enough energy can generate a dark spot on the film, exactly as photons. In this sense, you "directly see" the particles, without the use of light. Modern digital cameras are in principle able to detect particles passing through their sensors, although I do not suggest to make the test: it could permanently damage the sensors.
Finally: is it possible to "see" the particle using light? Interaction of a single particle with a light beam is possible, in principle, but, practically, it is never used in real detectors.
The double slit experiment for light or a beam of an accumulation of single particles footprints on a screen
Photons or particles of matter (like an electron) produce a wave pattern when two slits are used
This is the double slit experiment for electrons, one at a time, a former picture from this link.
The accumulation of electron footprints from a to e on the "screen" shows the wave nature in the interference pattern.
Their traversal through space to hit the "screen" is not seen, because to see a particle one would have to interact with it and change its direction and coherence with the beam.
