Heres curl example what works, and allows you to search multiple indexes:
curl 'http://localhost:9200/myindex1,myindex2/_search?q=*'
For single specific index:
curl 'http://localhost:9200/myindex1/_search?q=*'
To find find index names:
curl 'localhost:9200/_cat/indices'
And if you want to search all indexes:
curl 'localhost:9200/_search?pretty'