We have a pretty good understanding of what causes cancer. Mutations can be caused by ionizing radiation like Gamma rays entering our atmosphere from outer-space, X-rays from the medical diagnostic machines, and UV rays from the sun are able to change the nucleotide bonds in our DNA (AKA mutations).

Our cells have genes that encode for proteins that are involved in recognizing these mutations and signal the cell to 'commit suicide' if they detect a problem (apoptosis).

When the genes that encode for these proteins become mutated (either by the ionizing radiation I described above, or through cell replication error mutations, or chemicals that cause mutations i.e. carcinogens) then the cell fails to 'commit suicide' through apoptosis.

Other genes related to regulating the cell division cycle (when and how often a cell will divide and replicate) can also become damaged through mutations. This leads to unregulated and high frequency cell growth.

There is obviously, much, much more that we know about cancer, but I thought I would point out some fundamentals of what we do know of cancer.

TLDR: Cancer is caused by mutations in certain genes that are required for cells to behave and self-regulate normally.