The most simple query, which matches all documents, giving them all a _score of 1.0.

GET /_search
{
    "query": {
        "match_all": {}
    }
}

The _score can be changed with the boost parameter:

GET /_search
{
    "query": {
        "match_all": { "boost" : 1.2 }
    }
}

Match None Query

This is the inverse of the match_all query, which matches no documents.

GET /_search
{
    "query": {
        "match_none": {}
    }
}