Of roughly two dozen galleries now in the district and its environs, half have opened in the last year, drawn in part by a glut of cheap space. A fistful came from New York or Europe, all vying for talent and clientele. And there will be more, like the blue-chip Hauser Wirth & Schimmel’s 100,000-square-foot complex coming next spring. Within a few minutes’ drive of one another, the galleries are beginning to give the area the urban cultural density that Los Angeles mostly lacks.