Science fact is often stranger than science fiction so I have an open mind. My thoughts are that Panspermia between Earth and Mars was highly likely due to regular large meteor impacts early in both planets existence. Also many bacterial extremophiles are extremely hardy. To me the idea of DNA spreading around the galaxy in comets and leading to new life is harder to accept. In order for new cells to reproduce the basic cell would need to be preserved. We are talking hundreds of light years distance, millions of years in a high radiation free space environment. However as I have said, science fact is often stranger than science fiction.