Question: when were galaxies discovered to be large separate structures in the universe
[{'answer': '1923', 'provenance': [{'wikipedia_id': '58935', 'title': 'Timeline of knowledge about galaxies, clusters of galaxies, and large-scale structure', 'start_paragraph_id': 19, 'start_character': 12, 'end_paragraph_id': 19, 'end_character': 16, 'bleu_score': 1.0, 'section': 'Section::::Early 20th century.\n'}]}]