Junk DNA
Junk DNA is all the DNA in a genome with no known function.
Sequences that are genes have function, but many other sequences are functional. For instance chromosomes contains origins of replication, which define starting points of DNA replication. Also some parts of the DNA outside the genes are now known to be promoters of transcription or other regulatory sequences that figure in the control of gene activity.
As more functions are found the amount of DNA we now call junk DNA will likely decrease. It is also believed that junk DNA contains a pool of now defunct genes and gene fragments, known as pseudogenes, that were cast aside during evolution. Some hypothesize that the existence of junk DNA, which is not under selection, may provide a basis for further evolution, and thereby in a sense be functional. Some other evidence suggests that junk DNA is accumulations from failed viruses.
As much as 95% of the human genome is now considered junk DNA. Other species may have much more DNA than humans. For example the onion has 12 times as much, and it is believed that the excess is explained by junk DNA and pseudogenes. By contrast, the Pufferfish has about 10 times less DNA, but has a comparable number of known genes. Therefore it seems that the ratio of functional and junk DNA differs widely per species.
It should be noted that junk DNA is a broader category than selfish DNA. The former is an expression of our ignorance, whereas the latter is a positive hypothesis which may or may not be descriptive of particular sub-sequences of a genome.
