Big question indeed. Jesus spoke of them pretty often and His words can be relied on. Everyone thinks of it sometimes.
When ‘in extremis’ (very difficult, or near fatal state) everyone thinks of God and mostly pray or curse God. I have heard that soldiers say: “There are no atheists (don’t believe in God) on the front line.” Like on a ‘two way rifle range’, (war), or in a broken down boat with a storm coming up fast.
Deep down I believe we all know they are there but we would rather not think about it just now. “I’ll think about it when I am older,” is common.
In Luke, one of the two criminals dying with Jesus said: “Jesus, remember me when you come into your kingdom.” Jesus replied: “Today, you’ll be with me in paradise.” Jesus knew he was sorry for his deeds. That’s very clear to me that there is a heaven (Luke 23:39-43).
On Hell, Luke writes a story Jesus told of a very rich man, who cared for nobody else, but he died at that same time as a diseased beggar, who sat at his gate begging for food scraps died too. In torment, the rich man called to ‘Father Abraham’ to send the beggar over to him to “dip his finger in water to put on my tongue for I am in agony here”. Abraham replied that “in life you had everything but cared nothing for others, and the poor beggar is now here. A great gulf separates us, and he cannot help you even if he wanted to.” (Luke 16:19-31).
A Bible quote: “… and He is a rewarder of those that diligently seek Him.” (Hebrews 9:11). Read a Bible or Google the quoted verses and you’ll find it an intriguing and often fascinating book. Begin with John’s account of the life of Jesus. The Gospel of John can be read in about 3-4 hours. There you can learn that heaven and hell are very real.