Simplify the following text for a Grade 4 native English-speaking person.
Original: Recent observations indicate that this expansion is accelerating because of dark energy , and that most of the matter in the universe may be in a form which can not be detected by present instruments , and so is not accounted for in the present models of the universe ; this has been named dark matter .
Simplified: Most of the matter in the Universe may be in a form which can not be detected by present methods . This has been named dark matter .