Before I address your question, let me just say it is genuinely awesome that you asked it. Part of the responsibility of a modern scientist is to consider the impact of our work on the larger system of which we are all a part. Normally that system refers to the planet Earth and its atmosphere. You have applied this thinking to the entire cosmos, and that is impressive because you understand the seriousness of pollution and are thinking very, very big. Thank you for this question.
Let's begin thinking about space pollution by remembering what we know about planetary pollution:
• On earth, human activity has placed various toxins and waste in the water, soil, and air.
• These toxins and waste have been proven to affect the health not only of plants and animals (including human beings), but also of important physical and chemical cycles including but not limited to land erosion, the carbon cycle, the nitrogen cycle, and the formation of the ozone layer.
• The direct disruption of the health of living things is an obvious issue; the disruption of natural cycles impacts the stability of earth's climate and hence, indirectly, the health of living things.
In your question, you ask how much we are polluting space by exploring it. We call humans' effect on the environment "pollution" when we contaminate it somehow and make it dangerous for ourselves. This requires that we know what belongs in our environment and what does not. If we extend our concept of the environment beyond earth, immediately we have a few questions:
• What "belongs" in space? I would say that our old, obsolete satellites do not strictly "belong" in space.
• Is our space junk harming, well, space? How would we know? See next question.
• Is "unpolluted" space somehow sustaining the health of living things on earth? We know that the sun's energy helps sustain life on earth, the moon's gravity creates tides, and the position of earth in the solar system (mediated by the gravity of all bodies in the solar system) allows for liquid water and a liquid iron core (which creates a magnetic field that protects our atmosphere from being blown away by solar wind). Apart from our spatial and energetic relationship with the other bodies in the solar system (our moon, the sun, the other planets and their moons, the asteroid belt, and the comets), I do not know.
• What about the health of living things elsewhere? A timeless question. But based on our own delicate life balance, a few things about unperturbed space are likely working to sustain life elsewhere in the universe.
• Could we as humans disturb the universe enough to destroy ourselves or others? Space is vast, and we are small. Or so we think. New astrophysics, instead of giving us a better idea of how far the visible universe extends and how old it is, is actually making observations that generate more questions than answers. (For instance, a star was recently observed which appears to be older than we thought the universe was...) All this to say, the prevailing wisdom of the mid-20th century used to be that humans are too small in comparison with the earth to meaningfully impact global systems. "The solution to pollution is dilution," we would say. Boy were we wrong. And we could be wrong again, which is why your question is so important.
I think, with our many satellites, rovers, and spacecraft, we could be unknowingly harming the chemical, gravitational, or energetic balance in space. For example, on probes which travel far from the sun, solar panels cannot provide enough energy to operate the instrumentation, and therefore nuclear batteries are used. These batteries are advantageous because nuclear decay is slow and sustained. They are disadvantageous because nuclear waste is potentially dangerous to life. This was a major consideration in the retirement of the Cassini spacecraft, which until recently orbited Saturn and returned useful insights about the composition of its large rings. In 2017, the fuel that Cassini was using to maintain its orbit around Saturn was running out. The craft had to be retired, but because its nuclear battery could harm the environment of Saturn, the probe could not simply be allowed to fall to the planet's surface and crash. Instead, NASA flight engineers planned a careful series of oblique flybys, wherein the probe got closer and closer to Saturn's atmosphere and eventually burned up in flight. (You can read more about it on the wikipedia page.
Just remember to use more accepted sources if you are going to write an essay about this!) This suggests to me that there is an element of pollution to human space exploration. How much, as I've hopefully explained, is hard to know. But it is definitely important. Great question.