When I was diagnosed with breast cancer in 2010, I was told by the mammography radiologist that I had dense breasts. This was the first time I had ever heard this, and I'd been getting mammograms for 10 years.
In fact, I had never heard of dense breasts before and asked him what he meant. He explained "Your breasts are really dense and so the mammogram isn't able to see if there are any other lumps or suspicious areas in them." I pressed for more information and he then explained that when breast tissue is dense, it's more fibrous instead of fatty which makes it very difficult to see anything in them, even with a mammogram.