Both 밖에 and 바깥에 means "outside".
* 집 밖에 / 집 바깥에 : outside the house
While 바깥에 only means "physically outside", 밖에 has additional uses like "other than", "in addition to". In negative sentences it makes the "only" construct.
- 그 밖에 다른 의견 있습니까? Is there any more thoughts / suggestions?
- 난 이것 밖에 없어: I have none besides these = I only have these.