Imagine praising somebody by saying, "Hey! I think you're great, just not as great as me! Love you!" It's sort of what our speaker is doing in "Dangerous Astronomy." Spirituality is expressed as a desire to praise the stars, and as the poem unfolds, the speaker mixes his feelings for the stars with his feelings for his wife and son. Not bad, except for when he gets jealous. It's as if we accompany the speaker on a spiritual journey through his good intentions, disillusionment, and then eventual realization and plea for forgiveness from a "Rough God."
The speaker uses the contrast of light and dark to show that a spiritual experience is full of contradictions and other confusing stuff, like emotions, which can be uncomfortable to experience. That's why God is, you know, "Rough."
Through the speaker's conversational tone, he is able to articulate the difficult and elusive experience of spirituality into a typical, domestic situation between husband, wife and son. Gee, Alexie, why you gotta be such a ninja?