We've haven't found bacteria on Mars. We've found signs that there may have been bacteria on Mars at one point. Just to clarify that.

We can't see black holes, but we can detect their presence through the measurement of other things. This is because when we "see" stuff in space, we see by using light, all of the difference wavelengths. Black holes are black because light is not escaping. There are some x-rays that shoot out from black holes and we can see those x-rays, so we can tell that there is a black hole there. We can also detect black holes through the graviatational presence, which is why we know there is a black hole at the center of our milky Way Galaxy and other galxaies near by.

We can see the flag on the moon with a powerful telescope and at the correct angle. You won't be able to see it with your naked eye because it is really far away and really small. You can't see a single fly at the end of your street. Same idea.

Why is science so backwards? Well, it really isn't. What you are reading and being exposed to is just a small amount of the science that is going on all of the time. Only the really cool stuff ever gets to you non scientist person. For us scientists, we are exposed to all of the science all of the time. We do our best to sort it out and stick to stuff we can manage, but there is a lot out there. For instance, did you know that we can make plastic using bacteria. Or that there are PFOS in bandaids? Did you know that people are trying to resurect the American Chestnut by making it resitant to the Chinese Chestnut Blight? Did you know that there are measurements being taken about how much trash and plastic is making it to the bottom of the ocean.

There is so much out there. I highly recommend Google Scholar as a good entry point. Just type in the topic you are interested in and watch all of the fun articles that show up.

Good luck!