When I to the URL http://localhost:9200/_cat/indices I get like around 20 indices where only 4 have been created my me. All are named like:
yellow open .marvel-20XX.XX.XX 1 1 50712 0 51.2mb 51.2mb
.
Would it be right if I delete those?
Those are indexes created my Marvel. You can delete them, if you don't want the history of your cluster Marvel offers you.