What do you mean by junk DNA?

Because they are sequence of messenger RNA (mRNA) that the promoter - first codon, failed to find a pair in ribosomes. Example, if the sequence of promoter is UGC in mRNA, it must find an rRNA with a codon having the sequence of ACG in order for the process to happen - however, it will still be useless if there is no AUG sequence for it is the start codon and the process starts here. But if the promoter of the mRNA failed to find a pair, then it will no longer be used in the process, and then now called "junk DNA".

These junk DNA are used to fill up our genome.