What Causes These Dark Patches On My Lawn

Question

The grass seems much weaker and often there are toadstools in the darker area

Answer 1

The "darker" areas are, in fact, much healthier than the surrounding "yellow" patches.

I see a battle taking place. Those dark patches house mycelium of a mushroom. It would be hard to say which without seeing the mushroom in question. Lawn mushrooms are almost always good for the soil. Tree pathogens are different, but this is not the case, here. These mushroom species tend to ward off and kill any offending microbial colonies within a lawn.

It would be wise to allow these mushrooms to mature, and help spread spores around by crushing flattened caps and mushroom butt ends into wet soil where none exist, already.

https://www.gardeningknowhow.com/ornamental/fungus-lichen/environmental-benefits-of-fungi.htm