Earth and solar system is going around the milkyway galaxy.Te are huge interstellar clouds,gasses,dust clou?

First, while the sun and its system is orbiting the center of mass of the galaxy, so is everything else in the galaxy. Nothing is standing still, waiting for the sun to run into it. There is, therefore, no reason to be concerned about the Earth moving through any clouds of any sort. Although the Sun and solar system are moving relative to other objects within the galaxy, the distances involved require huge amounts of time to traverse (on a human scale).

Second, there are no "positron clouds" wafting about anywhere in our galaxy, except in the testing chambers of particle accelerators here on Earth. Antimatter does not exist in large concentrated quantities in the universe. Regular matter won out long ago.

As far as we can tell now, we are not on a collision course with anything and are "safe" for the next several hundred thousand years at least. In that time, we, as humans, will either wipe ourselves out, or develop the technology to overcome anything that could harm our civilization.

Considering how far we've come since my grandmother was born in 1890, I've got no fears that humanity is going to survive everything the universe can throw at it.

If the question is what will be our fate. I would say cancer and/or massive organ failure due to our DNA being melted by the high energy particles given off from matter anti-matter interaction.

For those mole-people that live below ground and somehow escape the battering? I guess they get all our stuff.

But if I may hazard, I think the answer to the implied question is...

The event cannot happen because almost all of the anti-matter in the universe was completely used up in the creative moments of the Big Bang leaving only a small minute amount of regular matter left to make the universe. ((Everything we are is just scraps left over... Vishnu was right!))

Also the cloud of positrons flowing into the path of our solar system would meet the outstreaming of our sun many light hours away from our planet anyhow, and so if the polarized solar wind did not shove the cloud aside like same-poled magnets then the electrons would get their chance at a battle royal.

So maybe the answer would be a fantastic light show?